The present participle of grump; complaining in a bad-tempered or grumpy manner, or the act of being grumpy.
From grump (to complain or act grumpily) with the present participle suffix -ing. Grump itself may derive from the onomatopoeia of growling sounds combined with influence from bump or lump.
The -ing suffix is one of English's superpowers—it turns any verb into a noun or adjective, so 'grumping' can mean the act of complaining AND feel like an adjective describing the action, showing how flexible our grammar is.
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