Alternative form of you could have fooled me

A mineral or other substance often mistaken for gold; mainly iron pyrite. / (figuratively) Something worthless that deceptively seems valuable.

(Britain, idiomatic) A cricketer (from the white flannel trousers they traditionally wear).

A person subjected to a practical joke on April Fools' Day. / A practical joke played on April Fools' Day.

present participle of fool around

(chess) The quickest possible checkmate in the game of chess.

(intransitive) to play the fool

(idiomatic) To behave in a foolish or comical manner. / (idiomatic) To pretend/feign ignorance.

(intransitive, idiomatic) To engage in frivolous behaviour; to monkey around. / (intransitive) To waste time. / (intransitive, informal, idiomatic) To engage in casual or flirtatious sexual acts.

A cap, usually brightly coloured with bells and tassels, as worn by a court jester or fool.