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English word grumpy pants comes from English -pants, English grumpy (Unhappy, dissatisfied and/or irritable.)
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-pants (English)
(informal, usually, pejorative) Used with adjectives ending in -y to form nicknames based on a negative quality of a person.
grumpy (English)
Unhappy, dissatisfied and/or irritable.
grumpy pants (English)
(countable, informal, sometimes used attributively) A person who is currently in a bad mood or is habitually cranky.. (uncountable, informal) A notional pair of pants worn by someone in a bad mood.