grumpy pants noun2 •(informal) A notional pair of pants worn by someone in a bad mood.We asked him to take off his grumpy pants and put on his big-boy pants. •[before a noun] (informal) Synonym of grump (“person who is currently in a bad mood or is habitually cranky”)."You fish, pal. Old grumpy-pants doesn't own the ocean!" — 1954
