Green’s Dictionary of Slang

five, esp. as £5 note.

[1848		Albert Smith Idler upon Town 55: A new and favourite ballad, a wedding-ring, an amusing puzzle, a five-pound note, and this pith tumbler. Hallo, my little man! there he is – heads up, Jack’s alive! Now who’s the lucky buyer of the last half dozen?].
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 6: Jack’s alive: £5 note.
1938		F.D. Sharpe Sharpe of the Flying Squad 331: jack’s alive (a) : A five-pound note.
1959		F. Norman Fings I i: Betcha a jacks yer don’t do a short time between yer.
1962		F. Norman Guntz 81: ‘Spend every penny’ [...] is what he told me to do, and then borrowed a jacks.
1977		G.F. Newman Villain’s Tale 73: Yeah, just let me nick a jack’s here first.
1981		(con. 1900–30) A. Harding in Samuel East End Und. 282: Jack’s alive – Fiver.
1989		J. Morton Lowspeak 78: Jack [...] five pounds. Usually used in the plural, from the rhyming slang, Jack’s alive = 5.