When a man is urinatng in a public urinal on his own and he attempts to urinate an equal amount in each urinal by sidestepping like a crab while using hands to control urine distribution.
"I went into the urinals and there was no one there so I did a bit of crabbing"
"My attempt at crabbing was ruined by someone coming into the toilets and spotting me mid-act, they gave me a pretty dodgy look"
Most of the time this is a Female by herself at the bar, sometimes she brings a friend. She sits there waiting for anything free; cocktails, shots, money, drugs, food... posting selfies to social forums, flirting, playing man like violin.
"Have you seen Lindsay?"
"It's Friday night, she's probably crabbing at the bar."