Green’s Dictionary of Slang

grease n.3

[abbr. greaser n.1 (6)]

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

[UK]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.

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