Green’s Dictionary of Slang

1. a fight.

1933		(con. WWI) A.E. Strong in Partridge Sl. Today and Yesterday 287: I believe we are in for a big smack-up.
1988		(ref. to 1900s–10s) McGill Dict. of Kiwi Sl. 104/2: smack-up a fight, c.1906; a battle in WWI.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. a crash, an accident.

1935		F. Wead Ceiling Zero Act I: Next time we have a smack-up, you’ll make the trip alone.
1959		G. Slatter Gun in My Hand 45: I was in a smack-up in the car.
1973		L. Shecter On the Pad 98: Civilians can pick up twenty dollars [...] if you’re lucky enough to run into a smack-up between two Cadillacs.
1998		M. Newall ‘Sir Gawayne and the Grene Knight’ in Indep. Weekend Rev. 26 Dec. 1: Contraflowe, roadeworks and a smacke-up by Stoke.