Green’s Dictionary of Slang

(orig. US gay) of a woman, to masturbate; thus jilling, female masturbation.

1989		J. Morton Lowspeak 80: Jill off – female, particularly lesbian, masturbation.
2001		Indep. on Sun. Life Etc. 29 July 11: Smith [...] gets one of the biggest cheers of the night when she mimics jilling herself off.
2004		‘Suzy Suburbanite’ 🌐 Tristan Butt Plug – When I’m setting aside some quality jilling time, then this beastie is the first thing that gets stuck anywhere.
2005		www.libidomag.com 🌐 All his [Roy Stuart] now-familiar elements are here: photo essays set outdoors and in lush indoor setting; a penchant for peering at panties and peeing; outdoor exhibitionism and lesbian fingering; multiple person encounters and single girl jilling.
2021		asstr,org 🌐 ‘[W]e have decided to have a little competition [...] a jill-off. The 2 girls will get on a table and masturbate to an orgasm 3 times’.