Green’s Dictionary of Slang

1. (Aus.) to water alcohol.

1907

Sun. Times (Perth) 14 July 4/7: Whiskey merely swells the list / Of our vile disorders, / So this fine philanthropist, / ‘Break It Down for Boarders’.

2. (Aus./N.Z.) to make lighter.

1945		R.S. Close Love me Sailor 175: Not my fault the old man logged you, Mister Samuals. I’ll do my best to break it down with the owners.
1984		Partridge DSUE (8th edn) 132/1: C.20.

3. (US campus) to relax.

1987

Eble Campus Sl. Spring 1: break down – take it easy, relax: I’ve had a long week – two exams and a paper. This weekend I’m going to break down.