Green’s Dictionary of Slang

jump-up n.

1. (Aus.) a paste made of flour, water and sugar [it jumps in the pan when boiling].

[Aus]W. Burrows Adventures of a Mounted Trooper 101: A young colonist [...] was in the habit of giving the blacks a feed of ‘jump-up’ [...] this stuff consists of flour and water boiled into a paste, and sugar put into it, and from the bubbles rising to the surface when boiling, they call it ‘jump-up’.
[Aus]Bulletin (Sydney) 8 July 24/3: He told the wallaby-trackers to mix a teaspoonful of salt and a teaspoonful of ‘jump-up’ into two pannikins of flour, and, after carefully stirring the dry ingredients, to mix in one pannikin of lukewarm water, knead the whole into a stiff dough, place on white-hot embers, cover with same, and bake one hour.
[Aus]Baker Popular Dict. Aus. Sl. (2nd edn).

2. (Aus.) the witness box.

[Aus]‘Rolf Boldrewood’ Colonial Reformer I 75: Take the book in your right hand, as they say when you are in the ‘jump-up’.

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

[UK]F. Norman in Encounter n.d. in Norman’s London (1969) 60: Well, if you must know, I got my first [conviction] for a jump up (stealing a lorry), and one for a blag (wages snatch), and another for tooling some flash tearaway (cutting a geezer with a razor).
(con. 1950s) M. McGrath Pie & Mash 168: The lads were doing the stealing, called jump ups because they used to jump up on the back of lorries and nick whatever.

4. (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].

[UK]T. White Catch a Fire 138: The various outdoor locations in Trench Town [...] where the ‘Sounds’ held their jump-ups.
[UK]Indep. Traveller 24 July 4: Don’t miss the ‘jump-up’ – fantastic local food and all the drinking and dancing you can take.
[UK]R. Antoni Carnival 64: Rudder had canceled [...] he was playing more than one pre-carnival jump-up. [Ibid.] 65: It was packed with raucous, bawling, thoroughly happy West Indians. [...] We jumped-up through Charlie’s Roots’ warmup.

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

[UK]J. Morton Lowspeak 84: Jump up [...] sexual intercourse.
[UK]J. Cameron Hell on Hoe Street 102: A few Pernods and a jump-up later.

In compounds

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

one who steals from lorries, trucks etc.

[UK]F. Norman Stand on Me 9: Tealeaves, conmen, jump-up merchants, and lay-down merchants.
[UK]D. Powis Signs of Crime 190: Jump-up man Lorry hijacker (one has to ‘jump up’ on to the rear of a lorry).
[UK]J. Morton Lowspeak.