Green’s Dictionary of Slang

1. (Aus.) the beginning, e.g, of a horserace [var. on jump-off n.].

1919		Newsletter (Sydney) 14 Oct. 7/3: Even then a fast beginner is liable to meet with bad luck in the jump out.
1922		Eve. News (Sydney) 1 Mar. 7/3: The mare faltered at the Jump out, and [...] she did not appear to have a winning chance.
1931		Toodyay Herald (WA) 17 Apr. 2/3: [photo caption] The Jump Out. ‘GO!’ — This field of nine runners leaping out of the barrier at Richmond.
1939		New Call & Bailey’s Wkly (Perth) 30 Mar. 3/4: Missed the Jump Out [...] drawn close to the rails at the barrier, he missed the jump-out and never had a chance of getting to the front.
1941		Baker Popular Dict. Aus. Sl.

2. (US drugs) the theft of a store of drugs.

2002

Simon & Burns ‘The Wire’ Wire ser. 1 ep. 6 [TV script] He know about the jump-out [...] all that shit.

3. (UK black/gang) a patrol manned by plainclothes officers.

2018-20

T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 Jump out – undercover police on patrol.