Question: In my own research, I found some references as far back as 1905 but does anyone have the origin of the phrase "Let that sink in"?

Answer: 
The relevant definition for *sink* in this sense in the OED is this:

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> To penetrate *into* (†*to*, *unto*, *through*), enter or be impressed *in*, the mind, heart, etc.
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Under this definition, the earliest entry is from a1300:

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> Sua sar þin sakes to for-thingk  
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>  þat soru thoru þin hert sink  
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> [Cursor mundi](https://quod.lib.umich.edu/c/cme/AJT8128.0001.001/1:4.1.171?rgn=div3;view=fulltext)
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I found that [the Middle English Dictionary](https://quod.lib.umich.edu/cgi/m/mec/med-idx?type=byte&byte=181222814&egdisplay=open&egs=181264699) has a lot of examples for this sense, under 3.(c).

As for the expression "let that sink in", a very similar expression can be found in this quote from 1385:

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> Lat oure sorwe synken in thyn herte  
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> [Chaucer: The Knight's Tale](https://sites.fas.harvard.edu/~chaucer/teachslf/kt-par1.htm)
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And in this quote from 1422:

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> In-to thyn herte let my wordes synke  
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> [How to Learn to Die](https://books.google.com/books?id=tGQiRWIlgy0C&pg=PA201&lpg=PA201)
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This same expression can be found much later, in this quote from 1798:

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> It is concerning a truth, as our Lord faith, Luke ix. 44. "Let these thinks sink in your hearts:" so we say, let this truth sink in your hearts.  
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> [The Ruin of Rome](https://books.google.com/books?id=tcI0AAAAMAAJ&pg=PA442)
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The expression "let [it] sink in your hearts" appears to be the origin of the shorter expression "let that sink in".

The earliest I can find for "let that sink in" is from 1837:

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> It appears, from the speech of the Leader of the Senate last night, that the only way in which the commission could supplement the work of the High Court would be in connexion with legislation which, while it might not infringe the provisions of the Constitution, might place one State at a disadvantage in relation to another. Let that sink in.  
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> [Parliamentary Debates](https://books.google.com/books?id=bCQEAAAAMAAJ&q="let+that+sink+in")
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After that, the next example I found is from 1895:

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> Both times I noticed his masterly use of the pause. It was as if he would say, 'There, let that sink in.'  
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> [A Memoir of George Higinbotham](https://books.google.com/books?id=8zlAAAAAYAAJ&pg=PA241)
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The idiomatic expression ***[sink in](http://www.dictionary.com/browse/sink--in)*** in the sense of *being understood* is quite old according to [The American Heritage Idioms Dictionary](http://www.dictionary.com/browse/sink--in)

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> * Penetrate the mind, be absorbed, as in *The news of the crash didn't sink in right away*. ***[Late 1300s]***
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***Let that sink in*** is a set phrase which derives from the above expression. Its earliest usages appear to be from the second half of the 19th century. I think the 1837 usage example present in [Google Books](https://books.google.com/ngrams/graph?content=let%20that%20sink%20in&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Clet%20that%20sink%20in%3B%2Cc0) is a false positive. 

[The Railway News](https://books.google.it/books?id=SgA1AQAAIAAJ&q=%22let%20that%20sink%22&dq=%22let%20that%20sink%22&hl=en&sa=X&ved=0ahUKEwjF27j3-enYAhVHxxQKHVYuBMMQ6AEINzAE) ..., Volume 44 - 1885

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> ***Just let that sink into your minds***. The Continental traffic via Dover and Calais and Folkestone and Boulogne has increased in much greater proportion during the existence of the Queenborough route than it did in the years before.
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