Green’s Dictionary of Slang

1. (also tear in) to throw oneself enthusiastically into a task.

1914		R. Tressell Ragged Trousered Philanthropists (1955) 217: It was therefore in the hope of being one of the favoured few that while inwardly cursing the rest for ‘tearing into it,’ everyone as a matter of self-preservation went and ‘tore into it’ themselves.
1915		Van Loan ‘The Extra Man’ in Buck Parvin 28: You tear in and save the lady.
1929		‘Henry Green’ Living (1978) 254: ‘Yessir’ foreman said ‘they’re tearing into it.’.
1938		D. Runyon ‘Little Miss Marker’ in Runyon on Broadway (1954) 297: The little doll tears into it very enthusiastically, using both hands.
1959		(con. 1940s) G. Slatter Gun in My Hand 92: Mugs up men. Get amongst it. Tear into the good stuff.
1972		K. Bonfiglioli Don’t Point That Thing at Me (1991) 27: We all three [...] tore into an enormous fry-up.
1998		R.G. Barrett Mud Crab Boogie (2013) [ebook] Warren and Debbie were [...] pulling cones from a bong on the coffee table while they tore into about a gallon of vodka.
1999		K. Sampson Powder 81: The Grams tore into their set with deranged energy.
2003		McGill Reed Dict. of N.Z. Sl.

2. to attack physically, or verbally.

1913		J. London Valley of the Moon (1914) 35: Hurray for the old lady! Look at her tearin’ into ’em!
1927		H.C. Witwer Classics in Sl. 28: He won’t rest ’til Hamlet has tore into said uncle and made him like it, for revenge.
1930		C.R. Shaw Jack-Roller 125: He tore into me for calling him ‘nigger.’.
1942		N. Algren Never Come Morning (1988) 55: The way you looked when you tore into Poor Charlie!
1951		D. Stivens Jimmy Brockett 277: Make it strong, brother. I don’t care how much ginger you put into it. Tear it into Regan.
1963		M. Spillane Return of the Hood 41: Penny tore into old Rudy Max and broke him up for not paying protection.
1974		G.F. Newman You Flash Bastard 69: Every villain and piece of scum who ever got a living from crime can, whenever he’s nicked, tear into the most practical, hardest working copper.
1985		B. Humphries Traveller’s Tool 35: We were tearing into each other like a pair of blue-tongued lizards.
1992		D. Jarman diary 29 Aug. Smiling in Slow Motion (2000) 208: They were venomously tearing into two pretty boys sitting across the restaurant.
2000		F. Mac Anna Cartoon City 40: He expected Lucy to tear into him. Instead he was pleasantly surprised when she came up and kissed him on the cheek.