Loose or baggy breeches or trousers, often worn in earlier centuries; absurdly large or ill-fitting pants.
From 'gally-' (a dialectal intensifier) plus 'gaskins' (loose breeches), a 16th-17th century term for wide leg coverings, popular in comic or derogatory descriptions.
Historical clothing terminology is hilariously specific—'gallygaskins' would make someone sound ridiculous precisely because it combined 'gally' (a mild insult) with 'gaskins' (already funny-sounding breeches), creating perfect mockery of exaggerated fashion.
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