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1.
a young man of great fertility, or, if used in the negative - an older one who has lost his virility.
Did you hear that Heather had an extremely physical affair with the 25year diving instructor she met on vacation? Five times a night apparently - he was a real sperm chicken.

Poor Jane and Mike are having problems conceiving - she's young and healthy, but Mike being over 50 is no sperm chicken anymore.
by narikin September 15, 2010