Green’s Dictionary of Slang

(Aus.) to masturbate; thus stropping n., masturbation.

1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Strop. To masturbate.
1999		G. Seal Lingo 126: Beyond the general Lingoisms for masturbation, such as STROPPING and WANKING, this act seems to inspire us to poetry.