form fitting white ribbed tank top worn by men; looks good on well-built fellas, pathetic on skinny fellas, and disgusting on fat beer bellied fellas

A jack off buddy (from 'cum' /'come', meaning 'semen', and 'companion'); a friend that you get together with for sessions of solo or mutual masturbation
Variant of: cumpanion (from 'companion' & 'cum'); jack off buddy & wank bud & JO buddy
Similar to: fuck buddy

A men's sleeveless undershirt.

I think the TV show "COPS" had a lot do with the creation of this word. Every time they showed a guy getting arrested for beating his wife he was wearing one of those sleeveless undershirts.

A tank top usually white

1. Men's close fitting, ribbed, sleeveless white cotton undershirt most commonly used before T-shirts came into vogue as undergarments. It is named wife beater after Marlon Brando's character, Stanley Kowalski, who wore one during much of the movie version of Tennessee Williams' "A Streetcar Named Desire." Stanley proved to be a sloppy, drunken, ill-tempered brute who beat his wife.
2. Slang name in Britain for a Belgian lager, Stella Artois, mostly because of the name, Stella, after Stella Kowalski, the victim of a beating from her husband, Stanley, in the aforementioned Tennessee Williams' play, "A Streetcar Named Desire," but also because of the high alcohol content which could possibly lead one to wife beating were he so pre-disposed.

A white sleaveles t-shirt also referred to as a muscle shirt or singlet.

In the UK, originally the white string-vest, but most vests/tank tops get reffered to as Wife-Beaters.
The oniginal example was so called due to the fact that the main type of person who wears string-vests was usually a fat, wife-beating, alchoholic.