Green’s Dictionary of Slang

fighting; also as v.

1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 159: ‘Hold my hat, Jim, I’ll be into him,’ i.e., I will fight him.
1873		Sl. Dict.
1904		Le Poer A Modern Legionary 23: [A]t any moment thirty or more men may be, to use an expressive phrase, ‘into’ one another with [...] anything lying handy that will do a man damage.
1918		S. Bourke & Mornington Jrnl (Richmond, Vic.) 11 July 2/8: I into ’im with a knuckle duster on the bunch of fives.
1936		Western Champion (Qld) 12 Dec. 3/2: I pulled off me coat and into him.
1951		D. Stivens Jimmy Brockett 62: When a smart alec called one of my ushers a poofter one night I was into him like a shot. I hit him on the chin and he went stern over appetite.
1959		G. Slatter Gun in My Hand 150: Crash into a tumbled heap on the landing and they’ll be into me. Scrag me.
1977		in K. Gilbert Living Black 131: When the Inspector went, didn’t he into me!