Green’s Dictionary of Slang

best friend n.

SE in slang uses

In phrases

be your own best friend (v.) [note Woody Allen’s line in his film Annie Hall (1977) ‘Hey, don’t knock masturbation! It’s sex with someone I love.’]

to masturbate.

2002		‘xpi3’ ‘Male & Female Masturbation Terms’ at Spankmag.com 22 Oct. 🌐 Being your own best friend.
2012		(con. 1980s) I. Welsh Skagboys 32: They go hame tae bed wi [...] a fistful ay that best friend whom they’ve let doon yet again.

wife’s best friend (n.)

the penis.

1884		Randiana 95: Drawing back my foreskin until my best friend’s topnut stood out like a glistening globe.
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 20: I’ve got to shake hands with the wife’s best friend!!!
1983		Monty Python ‘Meaning of Life’ 🎵 Your Piece of Pork, your Wife’s best friend, / your Porky or your Cock.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 52: Wife’s Best Friend Penis.
1991		Eble Campus Sl. Fall.
2014		http://theresmoretolifethanheavenandearth.wordpress.com 12 May 🌐 its allways fashioned me about men and the piece of equipment thay call there best friend.