Green’s Dictionary of Slang

a joc. term meaning to lean against, to support.

1845		G.W. Harris ‘The Knob Dance’ Spirit of the Times (N.Y.) XV July in Inge (1967)) 47: Two tall fine lookin galls was standin in the door, face to face holdin up the door posts with their backs, laffin.
1910		Bulletin (Sydney) 6 Oct. 11/1: He was holding up the verandah post of a pub and weeping bitterly.
1919		Aussie (France) 10 Jan. 12/1: We were standing outside the billet holding up the wall, sucking pipes and blowing smoke-rings, wondering if the Creator of man meant him to smoke, and, if so, why he didn’t plant a smoke stack in his nut.
1959		D. Hewett Bobbin Up (1961) 9: A group of bodgies, holding up the front of a milkbar.
1968		R. Gover JC Saves (1979) 665: Here is, my ole man. Holdin up the hotel, waitin on me.