Present participle of 'grouse'; the act of complaining or grumbling.
From 'grouse' (to complain) plus '-ing' (Old English gerund suffix). The present participle form, creating a noun describing the ongoing action of complaining.
Military folklore is full of grousing—soldiers' letters and diaries show that grousing was actually a form of psychological relief, a way to maintain morale by verbally processing hardship.
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