A castrated cat, or a castrated animal of certain other species.
From Old French 'Gibert,' a personal name that became a generic term for a neutered cat (similar to how 'tom' means a male cat). The term appears in Middle English texts.
This word is a perfect example of how animal nicknames become common nouns—'gibbert' for a neutered cat worked the same way 'tomcat' did, except this version basically lost the popularity contest and nearly vanished from English!
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