Green’s Dictionary of Slang

thrap v.

[dial. threap, to beat, to flog]

to masturbate; also as n.

[UK]Roger’s Profanisaurus in Viz 87 Dec. n.p.: thrap v. To masturbate furiously; to give it six nowt on the Right Honourable member for Underpants.
[UK]N. Griffiths Stump 4: Only easy thing to do with one hand is have a thrap.