(idiomatic) A sum of money, often relatively small in amount, kept in reserve to use for impulsive, frivolous purposes.

Demented or crazy.

(intransitive) To become insane. / (intransitive, informal, typically followed by "and" and a second verb) To do something extravagant.

plural of mad scientist

(slang) Demented, crazy.

A lunatic; a highly eccentric person.

plural of mad hatter

(obsolete) eggplant

plural of mad-apple

(idiomatic) extremely angry