Green’s Dictionary of Slang

(US) a bad-tempered person.

[1793		J. O’Keeffe London Hermit (1794) 15: He’s a very cross crusty grumps].
1835		‘Cock A Doodle-Doo!’ in Fanny Hill’s Bang-up Reciter 15: Poor Goody Grumpus lost her cock.
1861		G. Meredith Evan Harrington III 280: Mama displeases me in consenting to act as house-keeper to old Grumpus.
1887		P. M’Neill Blawearie 85: ‘Ods bods,’ ejaculated Will Hood, ‘here comes that great grumpus crying like a wane.’.
1964		P. Terson Night to Make the Angels Weep (1967) I viii: We’ll go on our botany expedition in the morning and leave old grumpus to get his own dinner, shall we?
1967–8		in Baker et al. CUSS.
1970		F. Collymore Notes for Gloss. of Barbadian Dial. 58: Grumpus-back. A surly person.