Means without. Used in text and IMing. Some people don't know what it means, but it's easier to figure out if you know what w/ means.
ih8every189: wut do ya wnat from taco bell?????
iluveverythinexceptu14: taco w/o cheese
iluveverythinexceptu14: taco w/o cheez
iluveverythinexceptu14: I SED GIMME A FUCKING TACO W/O CHEEZ!!!!
ih8every189: WTH is a 'w/o'?
iluveverythinexceptu14: 'w/o' means 'WITHOUT' DUMBASS!!!!
ih8every189: WELL SORRY IM NEW 2 THE WHOLE TEXT TALK
Short way of saying "with out"
I am going to the store w/o my wallet.