1) Someone who beats his wife is a "wifebeater" but this isn't the main definition of the word when used to define an article of clothing.
2) The main definition for Slang Urban purposes A type of sleeveless
undershirt, of the kind that used to be worn by almost all men in
the 50's. If you watch the HBO Drama
The Sopranos, you'll see Tony (the lead character) wearning this type of undershirt.
Also has associations with
slovenly, unkempt, sloppy, etc. Someone who just woke up and hasn't
combed their hair or washed up and appears at the door in a wifebeater shirt, scratching themselves, is a stereotypical picture that comes to mind.
This sleeveless undershirt with a deep
round neck and deep arm openings has acquired the connotation of an article of clothing worn by someone who beats his wife -- perhaps because of the unkempt, slovenly nature of the garment when worn alone on a middle aged man with a gut, sitting in an easy chair, scatching at himself and demanding to be waited upon lest he smack around the person waiting on him.
3) The same shirt, worn as daywear or nightwear by slender young women as a kind of
fashion statement.
Meg Ryan has worn them in movies, as have other stars/starlets -- not typically worn by a well-endowed woman, it is considered cool for a woman (who can get away with it and looks good) to wear this type of undergarment as an outergarment.
Note: It wouldn't be called a wifebeater when worn by a woman, for obvious reasons. It would just be another instance of a woman wearing typically male clothing and looking better in it than the guy does. ;)
There goes Anthony answering the door in his wifebeater; you'd think with the gut on him that he'd realize it doesn't look good.
"Take that off right now! You look awful; like a wifebeater/gangster or an unkempt homeless person in that greying old sleeveless
undershirt!"
She's slender and
in good shape; that slightly
snug sleeveless undershirt (wifebeater) she's wearing looks great on her.