Green’s Dictionary of Slang

apply v.

SE in slang uses

In phrases

apply lawyer foot (v.) [the image is of a foot that, like a lawyer, helps one ‘get away’]

(W.I.) to run away.

1956

cited in Cassidy & LePage Dict. Jam. Eng. (1980).

apply (the) lip gloss (v.) [the image of vaginal secretions coating the labia]

of a woman, to masturbate.

1998		C. Williamson ‘Lip service: euphamisms for female masturbation’ 🌐 There are still many female masturbation euphamisms listed on the pages above that are quite remarkable, delightful, exciting for their leap between the ordinary and the taboo-titillating sexual. Applying lip gloss, Checking the Oil, Doing something for my chapped lips.
2001		moxie.ca 🌐 Yes, May was Masturbation Month, kiddies, and in honour of this great event [...] allow us to present you with a detailed list of euphemisms for female masturbation, at least a few of which we think are clever. We dare you to try to use each and every one of them [...] Applying the lip gloss.