Flighty

Flight"y (?), a. 1. Fleeting; swift; transient.

The flighty purpose never is o'ertook,
Unless the deed go with it.
Shak.

2. Indulging in flights, or wild and unrestrained sallies, of imagination, humor, caprice, etc.; given to disordered fancies and extravagant conduct; volatile; giddy; eccentric; slighty delirious.

Proofs of my flighty and paradoxical turn of mind.
Coleridge.

A harsh disciplinarian and a flighty enthusiast.
J. S. Harford.