Young roosters; male chickens that haven't yet reached full maturity.
From cock (rooster) + -erel (a diminutive suffix from Old English, making it 'little cock'). This term has been used since Middle English to distinguish young male fowl from adult roosters.
A cockerel is essentially a teenage rooster—not quite grown but starting to develop the attitude! The diminutive '-erel' ending is the same one in 'mackerel' (a small fish) and appears in many Old English animal names to show smallness or youth.
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