sixty, n. — Green’s Dictionary of Slang

sixty n.

1. (UK Und.) a 60-day (two-month) jail sentence.

1865

Leaves from Diary of Celebrated Burglar 73/1: He’s only oot twa days frae doing a ‘sixty’.

2. (N.Z. prison) a 60mg morphine sulphate tablet.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 167/1: sixty n. a 60mg morphine sulphate tablet.

sixty-eight (n.) [play on sixty-nine n. (1)]

fellatio, i.e. ‘you suck me and I’ll owe you one’.

1972		B. Rodgers Queens’ Vernacular 182: sixty-eight 1. mutual fellatio wherein only one participant reaches climax.
1982		Maledicta VI:1+2 (Summer/Winter) 126: They object to […] Butch, French anything, Femme, Stud, Submissive, Dominant, Hermaphrodite, and Soixante-neuf – which is 69, in case you didn’t know; 68 is ‘you suck me and I’ll owe you one.’.
1997		Solar Project ‘Zeitgest’ 🎵 on ...in Time [album] You sit on my face, I dine at your Y / Blow job, gob job, sixty-eight / You feed your face and eat my meat / My fist into your Dead End Street.
1999		Roger’s Profanisaurus in Viz 98 Oct. 25: sixty-eight n. Oral sex undertaken on the understanding that the favour will be returned. As in ‘’Ere, give us a sixty-eight and I’ll owe you one.’.

sixty-six (n.) [play on sixty-nine n. (1)]

(Aus.) anal intercourse.

1966		Trimble 5000 Adult Sex Words and Phrases.
1972		B. Rodgers Queens’ Vernacular 89: anal intercourse [...] ninety-nine or sixty-six (the numerals 99 and 66 serve as ideographs in showing the whole story of two men going at it).

like sixty (adv.)

see separate entry.