Green’s Dictionary of Slang

bung-up n.

[bung v.1 (3) on model of SE leg-up]

the act of helping someone mount a horse, climb a wall, etc.

[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor III 133/1: Jerry [...] offers to give him a ‘bung up,’ and chucks him right over the pony’s back on to the ground.