Green’s Dictionary of Slang

a member of a motorcycle gang.

1963		Films & Filming 9:3 22: The lonely sailor’s pinup, the whistle-worthy object of a ton-up boy’s admiration [...] a working-class rather than a middle- class ideal.
1965		Manchester Guardian Weekly 22 Apr. 6: Sunning themselves by the warm walls were all varieties of layabouts and modern youth beatniks, mods, and ton-up boys.
1971		J. Mandelkau Buttons 30: I hing around in the background with the ton-up boys.