Alternative form of you could have fooled me

(idiomatic) A state of happiness due to illusion or false hope.

(historical, in medieval Europe) A festival, based on the Roman Saturnalia, celebrated near Christmas.

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.

(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.

(idiomatic) A foolish undertaking, especially one that is purposeless, fruitless, nonsensical, or certain to fail. / (idiomatic) Such an undertaking, assigned as a prank.

present participle of fool about