Green’s Dictionary of Slang

1. (US, also grandma gear) the lowest gear of a vehicle [the image of a slow-driving grandmother].

1941		‘Army Sl.’ AS XVI:3 166/1: grandma. Low gear.
1966		‘Tom Pendleton’ Iron Orchard (1967) 11: Come on, ye wore-out ol’ sonofabitch! Take a country mile t’git you outa grandma!
1973		J.L. Gwaltney Destiny’s Chickens 10: He throwed ’er into Grandmaw [HDAS].
1984		K. Weaver Texas Crude 9: ‘Grandma’ is the lowest, therefore the slowest, gear in a truck transmission.
2002		C.R. Stekelenburg Burton Murder 13: Just leave the floor shift lever in compound low or grandma gear [...] That is the slowest most powerful gear so you shouldn’t get stuck.

2. (US gay) an old(er) homosexual; thus grandpa, an ageing lesbian.

1972

B. Rodgers Queens’ Vernacular 25: middle aged homosexual [...] grandma [Ibid.] 71: the aging lesbian [...] grandpa.

3. (N.Z. prison) a veteran female prisoner.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 81/1: grandma (also grandnan) n. (in a women’s prison) an old inmate.