The most notorious out the trio (player, manwhore, walking sti), the term is a derogatory term for an extremely promiscuous man. The walking sti is literally what the name implies, as this subject will usually have slept with so many women that the odds of having contracted some type of sti are most likely very high. The complete opposite of the player, the walking sti values quantity over quantity. His reputation is usually bad among both males and females, and unless dramatic changes are made, will most likely never establish a meaningful relationship with women, as all he will want of them is sex. This term is best used for the select males who hook up with women for one night and then never speak to them again, or otherwise treat them poorly.
G1: OMG, WHY ISN'T BOB TALKING TO ME? I THOUGHT WE MADE A CONNECTION LAST NIGHT...
G2: No honey, Bob is a walking sti. You wont be hearing from him again, and you might also want to consider checking yourself out. There's no telling where he's been.