sleeve tattoo (or tattoo sleeve) is a large tattoo, or a collection of smaller tattoos, that has a unified theme, that covers most or all of a person's arm, 
What'd a sleeve tattoo?
sleeve tattoo has a unified theme, whereas an arm covered in tattoos may have many tattoos 
OK why he need for tattoo in the first place?
Tattooing has been practiced across the globe since at least Neolithic times, as evidenced by mummified preserved skin, ancient art
Wow! It really dates back!
to the tattoo's size similarity in coverage to a long shirt sleeve on an article of clothing. In this manner, the term is also used as a verb; for example, "getting sleeved" means to have one's entire arm tattooed. The term is also sometimes used in reference to a large leg tattoo that covers a person's leg in a similar manner.
Half-sleeves or quarter-sleeves are tattoos that cover only part of an arm, usually above the elbow, but can also be found
Sleeve tattoo
A sleeve tattoo (or tattoo sleeve) is a large tattoo, or a collection of smaller tattoos, that has a unified theme, that covers most or all of a person's arm, usually from shoulder to wrist.
There is a difference between an arm covered in tattoos and a sleeve tattoo. A sleeve tattoo has a unified theme, whereas an arm covered in tattoos may have many tattoos of different styles that does not have an overall unity.
The term "sleeve" is a reference