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'.