Green’s Dictionary of Slang

bread knife n.

one’s wife.

2002		D. Shaw ‘Dead Beard’ at www.asstr.org 🌐 I make a mental note to be extra wary in future about keeping a couple of relationships dead hush-hush from her, otherwise my bread knife might be giving me the treatment with one.
2003		B. Dark Dirty Cockney Rhy. Sl. 30: I’ve got to get home or else the bread knife will cut me up.