Green’s Dictionary of Slang

a mild euph. for Christ! excl.

1866		Wild Boys of London I 85/1: ‘Crummy,’ thought Sam; ‘shouldn’t I like to speak to her.’.
1899		‘Tom Bluegum’ Backblocks’ Parson 187: By crummie he’s a queer joker.
1914		Bulletin (Sydney) 3 Sept. 22/4: [A]n every time the same thing happened and then by crummies I took a tumble.
1941		G. Kersh They Die with Their Boots Clean 4: We was going to die in proper order, by crummit we was.
1942		Whizzbang Comics 76: ‘Crummy!’ cried the excited Billy.
1995		P. Cornwell From Potter’s Field (1996) 292: Well, for crummy sake, why haven’t you canceled it?