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Sometimes considered a term of affection, sometimes a homophobic slur when spoken. 'Thilly gooth' means "silly goose" but its usually spoken in an attempt of an overblown fake gay mannerism voice.

A perfect example is the character Big Gay Al from the TV show SOUTH PARK.
Sean... the things you say are tho funny you thilly gooth!
by Alpha Kennyding February 17, 2011