Green’s Dictionary of Slang

a phr. based on one’s childhood, or even earlier life, all of which mean a very long time ago: (Aus., …just a dirty look, …just a gleam/twinkle in your father’s eye, …still in/wearing short pants, …running up and down your father’s backbone, Aus.) when your mother was cutting bread on you.

1960		A. Buzo Front Room Boys Scene ii: I helped open up the bush, mate. I was out the back o’ Bourke when you were a dirty look .
1984		Partridge DSUE (8th edn) 1326/2: when you were... There are many c.pp. for ‘when you were a child, boy, baby, or even before that...’ ; a few are: when you were wearing short (or, intensively, three-cornered) pants: Aus.: since ca. 1920 (B.P.); when you were just a gleam (or twinkle) in your father’s eye; or, more directly, when you were running up and down your father’s backbone; [...] C.20; when your mother was cutting bread on you; when you were cutting bread and jam (I was cutting barbed wire): army: WW1.
2012		M. McBride Frank Sinatra in a Blender [ebook] He and Doyle went back to the days when Moses wore short pants.