Green’s Dictionary of Slang

flap v.3

[flap n.1 (4)]

1. of a man, to have sexual intercourse.

[UK]Farmer & Henley Sl. and Its Analogues.

2. to masturbate.

[UK]B. Aldiss Hand-Reared Boy 51: William, by his own account, ‘flapped himself’, as he called it, every night.