Green’s Dictionary of Slang

hold up v.2

[the pretence of ‘holding up’ the structure against which one leans]

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

[US]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.
[Aus]Bulletin (Sydney) 6 Oct. 11/1: He was holding up the verandah post of a pub and weeping bitterly.
[Aus]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.
[Aus]D. Hewett Bobbin Up (1961) 9: A group of bodgies, holding up the front of a milkbar.
[US]R. Gover JC Saves (1979) 665: Here is, my ole man. Holdin up the hotel, waitin on me.