Green’s Dictionary of Slang

sling shot n.

1. (US) a flick-knife.

[US]‘Iceberg Slim’ Pimp 137: I fingered the button on that sling shot in my ‘raise’.

2. as a form of under-garment [the shape].

(a) (US black) a sanitary towel.

[US]E. Folb Runnin’ Down Some Lines 197: There are expressions related to pregnancy, menstruation, birth control, and female hygiene: [...] sling shot, diaper (sanitary napkin); diaper the baby (put on one’s sanitary napkin); red Mary (menstrual period).

(b) (W.I.) skimpy underwear; a thong.

[WI]Francis-Jackson Official Dancehall Dict. 48: Sling-shot skimpy underwear.

(c) (US prison) male underwear, briefs; also attrib.

[US]N. McCall Makes Me Wanna Holler (1995) 172: Don’t wear slingshots. That’s the prison name for men’s briefs.
[US]G. Pelecanos Shame the Devil 76: I see this poster of a rapper [...] Wearing one of those slingshot swimsuits.

3. (US black) a Cadillac Eldorado [ety. unknown].

[US]E. Folb Runnin’ Down Some Lines 84: By far the most popular car among those I met was the Cadillac – variously referred to in the vernacular [...] by special model names, the B (Brougham), El D, Rado, slingshot (El Dorado).