Doing something big that will get you in trouble, but you don't care. This term derives from how John Hancock wrote his name the biggest on The Declaration of Independence.
John: I just banged eight fat chicks!
Bob: Your wife is going to be pissed!
John: I don't care! I love Hancocking!
Tim robbed a bank, walked across the street and started jerking off in front of everyone. The police were on their way. He didn't worry a bit. He's really great at Hancocking.