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.
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. ;)
"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.
The shirt has its name derived from the police shows when they track down an actual wife abuser from a local trailer park or run down neighborhood. The said individual, in most cases, is wearing this type of garment. Thus the name "wife beater"