The act of "measuring up" with another man. It happens when two men (usually drunk or high) will start to argue about penis length. It happens when one man pulls it out and lays it on a table. This is the challenge stage.
When the other man lays his out and both penises are measured, it is said that the winner has longcocked the loser. He will then collect his spoils, if there were bets placed.
"Old Harry was out longcocking at the pub last night. He won three drinks before the cops took him home."
"After Dave longcocked the police chief, he was never given another speeding ticket again. The chief kept his longcocking habit on the down low for a few months after that."