One who crows; a rooster, or a person who boasts or crows about something.
From 'crow' (to make a rooster's cry, or to boast) + '-er' (one who does the action). Standard English agent noun formation applied to both literal roosters and figurative boasters.
The word 'crower' for a person who boasts shows how agricultural life shaped English—roosters were so famously loud and proud that their very name became synonymous with bragging.
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