The original 'gangster lean' came from old gangster movies and may have even been taken from real life. When
fleeing (usually from the scene of a crime) and a car pulls up and you jump on the side of one of those old gangster cars like a 1937
Hudson Terraplane, and you lean on the outside of the car until you're far away.
From
Roman Polanski's 1974 timeless crime/mystery classic 'Chinatown'. Jack
Nicholson and
Faye Dunaway are told to get into a car by some thug, and Nicholson goes to beat him up while Faye takes the car and starts it towards Nicholson as he jump onto the car and does the 'gangster lean'.