Green’s Dictionary of Slang

to help, esp. in climbing up or over an obstacle, when one person either lets the other stand on their back or links their hands to make a ‘stirrup’ into which the climber can put one foot and boost themselves upwards.

1894		A. Morrison Tales of Mean Streets (1983) 151: It is true that this last did sometimes involve unpleasant exertion when the window was high and the boy heavy to bunk up.
1906		Sun. Times (Perth) 18 Mar. 4/8: He bunked our bunkers up with sucking pig.
1929		C.G. Gordon Crooks of the Und. 241: He prevailed upon me to ‘bunk him up’ upon the wall.
1965		W. Dick Bunch of Ratbags 70: Hey, Terry, bunk us up on the wall an’ I’ll ’ave a look over.