Green’s Dictionary of Slang

1. motorcycle riders (Rockers, as opposed to Mods); also collectively as the grease.

1971

J. Mandelkau Buttons 20: This gang was ordinary grease, or what most people called Rockers.

2. (US) a white working-class youth, a member of a hot-rod, motorcycle or juvenile gang.

1967

S.E. Hinton Outsiders 23: I’m a grease, same as Dally. He’s my buddy.