To take your flaccid penis otherwise known as your "willie" and smack someone with it. The most highly prized Toby Tanking is across the face and as a surprise. It is played as a game akin to tag. More common in overly masculine homo-erotic circles of men.
A: What happened to you?
B: I Toby Tanked your cat while it was napping on your desk.
A: Awe No.
B: Aye, it was a shit idea.