Uncertain. Possibly altered from grum, see there for more.
Meanings
(informal) A habitually grumpy or complaining person.
(informal) A grumpy mood.
If you're not taking Action to change the situation, either sit in a grump or Accept it. 2004, Toby Chapman-Dawe, Passport to Performance: Raise Your Business Performance!, page 29:
Uncertain. Possibly altered from grum, see there for more.
Meanings
(informal, intransitive) To complain.
But Murial Jane was now in a grumping mood[.] 1936, Norman Lindsay, The Flyaway Highway, Sydney: Angus and Robertson, page 69:
Grumping to himself as he made up his own bed, Bradly thought, "Have to wash that blanket; young blighter going to bed with all that mud on his feet." 1938, Norman Lindsay, Age of Consent, 1st Australian edition, Sydney, N.S.W.: Ure Smith, published 1962, →OCLC, page 72: