Green’s Dictionary of Slang

1. to break, to smash .

c.1911

H. Champion ‘Good Old Yorkshire Pudding’ [monologue] My wife once joined the Suffragettes [...] One night they diffed the windows and grabbed the bacon boiled.

2. (Aus./US) to have sexual intercourse with.

1993

B. Moore Lex. of Cadet Lang. 126: usage: ‘I’m getting there, mate, I’ll be duffing her before long’.

3. (Aus./N.Z.) to impregnate.

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxiv 4/4: duffed: Pregnant.
1988		McGill Dict. of Kiwi Sl. 41/1: duff to make pregnant, developed from British phrase ‘up the duff’, to be pregnant.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].