I'm attracted by the argument made here , that it's to do with trams, given that both "off his trolley" and "off his rocker" appear in print around the same time.
I also feel the fact that all the early citations are about "going off your rocker" or "being driven off your rocker" - rather than falling/running/getting - suggests it's the same as "going off the rails".
That said, rocker
at the time could refer to: chairs cradles rocking horses ore agitators rocker arms part of a coach or car Trevelyan's rocker a technical term in
numerous patents
Both words have continued to be overloaded: growing up, I associated "off your trolley" with shopping carts, and "off your rocker" with rock stars.
english.stackexchange.com/users/2895/Mark
Etymonline says : rocker "a rocking chair," 1852, Amer.
Eng., from rock (v.); [...] Slang off (one's) rocker "crazy" first recorded 1897.
So it's someone who's off his rocking chair.
The Phrase Finder also has a thread with people speculating about a different origin, but the conclusion is: The phrase relates to rocking chairs according to "The Dictionary of Cliches" by James Rogers (Ballantine Books, New York, 1985).
english.stackexchange.com/users/300/RegDwigнt
This is more speculation than anything, but it's somewhat stereotypical for older people back in the day to be sitting on rocking chairs and just passing the time,
right?
What if the phrase originally didn't mean to suggest that someone merely is crazy, but also outwardly displaying it?
A crazy person might feel the urge to get out of their chair, run around and act mad, whereas a more mild mannered one would just quietly rock back and forth and not cause a scene.
english.stackexchange.com/users/2953/Joost Schuur
The best answer I found: A more interesting topic than first meets the eye!!
The phrase first came to light around 1896, and there are 2 schools of thought as to its original reference and meaning: School one:
The "rocker" was the metal strip which held the long metal arms of electric trolleys onto their track.
I remember Halifax in the early 1960s still had them - pretty comical when the conductor had to get off in all kinds of weather to manoever the arm back onto its "rocker"!!
Here's a quotation from the first source below: "......
the first appearance in print of OFF ONE’S TROLLEY in 1896 (see quote below) was followed less than one year later in 1897 (see quote above) by OFF ONE’S ROCKER.
This, and the fact that electric trolleys were ‘off’ and running in the 1890s would seem to indicate that we might have more than just a coincidence here.
And it strikes me that falling ‘asleep’ in one’s rocking chair is a much more likely event than ‘falling off one’s rocking chair’ –
an occurrence seemingly too rare to warrant having an expression named after it – or the even rarer occurrence of somehow being ‘off the rocker’ on one’s rocking chair (whatever that might mean – it’s hard to visualize).
On the other hand, even by the 1940s when I would pass three different trolley lines on my mile or so walk to my elementary school every day (yes, kids actually walked to school in those prehistoric times), I often saw men working on a stalled trolley trying to get that overhead, spring-loaded pole, back onto the power line where it belonged – it had gone OFF ITS ROCKER.
"
School two (referred to above also): " Originally the phrase was specifically about what we'd now call "mood swings."
Now it means "to be crazy," but it used to connote a "normal" person who periodically, and unexpectedly, goes crazy.
"
Aw, he's just off his rocker--
it'll pass."
Just imagine the kind of person you'd expect to find "on a rocker.
"
An elderly person, the stodgy old grandma or grandpa
most every household has in residence (the expression dates from 1897, before nursing homes were so acceptable).
Most of the time they just sit and rock, hands folded, muttering quietly.
But every now and then a sudden agitation overcomes old Uncle Nancy, he jumps up and cavorts around the house.
Or runs off to the wars with his WWII [actually, WWI]
Home Guard helmet on-- "out of uniform" so to speak, like Hyacinth Bucket's old dad.
[from the hilarious British sit-come "Keeping up appearances" - still re-run on some USA tele channels!]
" - from source 2 below. Source(s): http://www.wordwizard.com/phpbb3/viewtopic.php?f=7&t=18057 http://forum.wordreference.com/showthread.php?t=96503 From: http://answers.yahoo.com/question/index?qid=20100831080317AAbDNVw
english.stackexchange.com/users/49312/Jacqie
'Chambers Slang Dictionary’ confirms the rocking chair origin.
It also gives this further definition of its use since the 1950s: in figurative use, acting excessively (though not necessarily madly)
The OED’s earliest citation is dated 1890.
english.stackexchange.com/users/12952/Barrie England
1765 is another possible first in another vein: Mother Goose and rocker association with lulling, or casting out of being lulled, one would suppose, might likewise be a metaphor for the passage forward from baby hood...or, possibly, back into earlier babyhood...and figurative need of lulling ...
in the sense, respectively, of mental/emotional maturation or regression.
If only the cited authors in above-cited instances of rocking-chair/rocker association with mental disturbance were alive to be asked (if they based their allusion on familiarity with the rhyme, Rockaby Baby (in the treetops...when the bough breaks down will come baby, [rocking] cradle and all), their answers might come closer to definitively answering the whence and wherefore. http://en.wikipedia.org/wiki/Rock-a-bye_Baby
english.stackexchange.com/users/28414/lex
It is a term of the industrial revolution.
A rocker arm is (was) used in internal combustion engines and in industrial looms.
It is a fragile part of the machine.
When the machine went "off its rocker" the machine had to be stopped immediately (not within seconds, that would be too long) or it would destroy itself and possibly those working around it.
english.stackexchange.com/users/47121/feralrom
The expression "off his rocker" is used to mean someone is mad (as in, bonkers mad, not angry mad). Does anyone know what a rocker is, and how being off one came to mean this?
Why "off his rocker"?
english.stackexchange.com
<phrases>
english.stackexchange.com/questions/6967
english.stackexchange.com/users/305/Brian Hooper
It is not fully clear where the expression "off his rocker" came from, but a likely explanation is that it relates to someone falling off their rocking chair, with a rocking chair being also known as a 'rocker'. This is also confirmed by the Chambers Slang Dictionary. The word may relate to trams or trolleys which appeared at the time it was first heard.  There is a suggestion that the word orginated from "rocking chair" and its first known slang use was in 1897.  In modern times, it means someone who is crazy but the term is suggested to originate from the name of a part of a combustion engine called a rocker which as a fragile part of the machine had to stopped immediately or it would destroy itself.
It is not fully clear where the expression "off his rocker" came from, but a likely explanation is that it relates to someone falling off their rocking chair, with a rocking chair being also known as a 'rocker'. This is also confirmed by the Chambers Slang Dictionary. There is a suggestion that this originates in trams and railways, and means "going off the rails" or going mad.
"Rocker" in this phrase refers to "rocking chair". It is generally agreed that a "rocker" refers to a rocking chair.
It may be related to "trams" as its usage appeared when "trams" or "trolleys" first appeareed. The connotation of the word has grown in time. Some sources suggest the word comes from a "rocker" or "rocking chair" The term originally may mot have denoted madness. It is a term from the industrial revolution relating to the a part of a combustion engine.
There is a suggestion that this originates in trams and railways, and means "going off the rails" or going mad.
It is generally agreed that a "rocker" refers to a rocking chair.