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

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

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

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

(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.

(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

(intransitive) to fool around

(intransitive) to play the fool

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