Green’s Dictionary of Slang

underpinnings n.

the legs.

[US]E. Bennett Mike Fink 9/2: Nothing like long under-pinins fur travel.
[US]N.Y. Dramatic News 5 Oct. 6/1: Do cigarette girls at work wear their dresses decollete at the bottom and show their underpinnings? [OED].
[US]A. Woollcott While Rome Burns 48: There he was at last with his underpinnings shot from under him.
[US]Wentworth & Flexner DAS 560/2: [...] Since ca.1850.