Green’s Dictionary of Slang

jump-up n.

1. [mid-19C–1940s] (Aus.) a paste made of flour, water and sugar [it jumps in the pan when boiling].

2. [late 19C] (Aus.) the witness box.

3. [1940s+] (UK Und.) hi-jacking a lorry and/or stealing its contents [jump up v. (2)].

4. [1950s+] (W.I.) a wild dancing party [orig. held as a funeral wake, but now in general use; note US jump-up/jump-up song, a lively song with ad hoc lyrics, often extemporized from various proverbial sayings].

5. [1970s+] (US black) sexual intercourse [jump up (and down) under jump v.].

In compounds

jump-up merchant (n.) (also jump-up man) [merchant n.]

[1940s+] one who steals from lorries, trucks etc.