Green’s Dictionary of Slang

the legs (as a support).

1848		E. Bennett Mike Fink 9/2: Nothing like long under-pinins fur travel.
1895		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].
1927		J. Woods ‘Sports Topics’ in Brooklyn Dly Times 13 May 2A/3: Sid will depend on his high-geared underpinning to waft him hither and thither.
1934		A. Woollcott While Rome Burns 48: There he was at last with his underpinnings shot from under him.
1960, 1975		Wentworth & Flexner DAS 560/2: [...] Since ca.1850.