A cut-off sleeved t-shirt, usually white in color, worn by people(usually) men when out in the outdoors, working out in the gym, or other instances when they think it is appropriate to show off their arms.
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"
"Let's go play some b. ball" "Ok, let me grab my wife beater."