madcap

Short definition from Wiktionary

Noun

1. 1
   A person who acts in a capricious, impulsive, or reckless manner.
2. 2
   A very foolish or irresponsible person; a fool, an idiot.
3. 3
   An insane person; a lunatic.

Adjective

1. 1
   Capricious, impulsive, or reckless; also, bizarre, crazy, zany; or foolish, idiotic.