Green’s Dictionary of Slang

1. a large and clumsy person.

1818		W. Perry London Guide 40: If a row is begun, you join in the phrases used, as ‘go it;’ [...] ‘now, d — n his eyes;’ [...] ‘go it my jumbo,’ or whatever may be said upon the occasion.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 107: Jumbo a clumsy or unwieldy fellow. ‘Go it, my jumbo,’ said to an ugly wallupping chap. Watermen to hackney-coaches, market-porters and others, who wear heavy patched-up habiliments are addressed with ‘My Jumbo.’.
1890		M. Williams Leaves of a Life 45: Jumbo and I, after going to the theatre, had supper.
1898		J.D. Brayshaw Slum Silhouettes 215: ‘Wot cher, Jumbo!’ ‘Wot cher, Fatty! Seed Cocky Brown lately?’.
1900		Bulletin (Sydney) 9 Jun. 24/1: It has been customary for jocks – some of them jumbos – to get on the scale, show that they weigh over (say) 10st., and then hop off without declaring how much they are over the required weight.
1914		E. O’Neill Abortion in Ten ‘Lost’ Plays (1995) 164: Come on, Jumbo.
1938		(con. c.1910s) S.H. Holbrook Holy Old Mackinaw 198: He came face to face with the mountainous Jumbo.
1949		A. Hynd We Are the Public Enemies 95: Chief of Police Carl Galliher – a big jumbo of a man.
1997		H.R.F. Keating Soft Detective 56: Jumbo heaved himself to his feet.

2. the Elephant and Castle public house in south London.

1909		J. Ware Passing Eng. of the Victorian Era.
1936		J. Curtis Gilt Kid 98: Got to go over to the Jumbo to see a bloke.

3. as an affectionate term of address.

1897		Mirror of Life 23 June 3/2: [W]e have sampled the weeds sent by ‘Jolly Jumbo,’ and found them excellent. Jumbo, we thank thee!
1919		E. Dyson ‘Hello, Soldier!’ in ‘Hello, Soldier!’ 85: ‘Cri, it jumbo.’ ‘Have a beer.’ / ‘Wot-o, Anzac; you’re a dear.’.
1966		(ref. to 1890s) J. Holledge Great Aust. Gamble 120: Back in the 1890’s a prominent sportsman and gambler [...] boasted the merits of his trotter Darktown to Sydney Paddock bookmaker ‘Jumbo’ Barnett.

4. (N.Z.) the buttocks, the posterior.

1988		McGill Dict. of Kiwi Sl. 64/1: jumbo backside, c.1945.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

5. a fool, simpleton.

1970

G.F. Newman Sir, You Bastard 98: Don’t want no jumbos stealing my glory.

6. in drug or drink use.