A stomach pain or ache, or to complain constantly about something.
A compound of 'belly' (stomach, from Old English 'belg') and 'ache' (pain, from Old English 'acan'). It's a folk term that's been used for centuries.
'Bellyache' is one of those wonderfully descriptive compound words that's pure English—nobody needs to explain which part of you hurts. As a verb meaning to complain, it captures the idea that endless complaining is annoying like a constant stomach ache.
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