Green’s Dictionary of Slang

flip off v.¹

to masturbate.

1972		B. Rodgers Queens’ Vernacular 115: to masturbate [...] flip it [around, off].
1985		‘Thommo’ Dict. Aus. Swearing & Sex Sayings n.p.: FLIP OFF — Male masturbation, very casual, almost a waste of time.
2002		‘Animal Farm’ at GalleryListing.com 🌐 I’ll bet he’s still flipping himself off! Not that there’s anything wrong with masturbation, of course. It’s just better to have a partner now and then.

In phrases

flip oneself off (v.)

(Aus.) to masturbate.

1955		‘No. 35’ Argot in G. Simes DAUS (1993).
1978–99		G.A. Wilkes Dict. Aus. Colloquialisms.
2002		‘Animal Farm’ at GalleryListing.com 🌐 I’ll bet he’s still flipping himself off! Not that there’s anything wrong with masturbation, of course. It’s just better to have a partner now and then.