ninety, n. — Green’s Dictionary of Slang

ninety n.

also 90 dog

[the curled tail, which resembles the number 9]

a pug.

1909

J. Ware Passing Eng. of the Victorian Era.

SE in slang uses

In compounds

ninety-nine (n.) [a play on the usu. sixty-nine n. (1)]

(Aus./US gay) anal intercourse.

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).

ninety-six (n.) [var. on prev.]

(US gay) homosexual anal intercourse.

1914		L.A. Times 14 Nov. II 8: He said that the Ninety-six Club was the best; that it was composed of the ‘queer’ people, that got together every week. I asked Lowe why they called it the Ninety-six Club, and he said someting about turning the letters around, before and behind.
c.1925		R. McAlmon Miss Knight (1963) 66: She [...] out of a now gayer mood and of an habitual bitchy gaiety, shouted across the aisle to a Germany boy she knew, ‘Oh you Suzie stoopantakit, I got your number. It’s – 96 – ain’t it? You know, dearie, I think yer queer’.
1949		‘Swasarnt Nerf’ et al. Gay Girl’s Guide 13: ninety-six: California term for reciprocal anal intercourse.