lu·na·tic 
(lnə-tĭk)
(lnə-tĭk)n.
1. A person who is affected by lunacy; a mentally deranged person.
2. A very foolish person.
adj.
1. Affected by lunacy; mentally deranged.
2. Of or for people who are mentally deranged.
3. Wildly or giddily foolish: a lunatic decision.
[Middle English lunatik, from Old French lunatique, from Latin lūnāticus, from lūna, moon (from the belief that madness was influenced by the phases of the moon); see leuk- in the Appendix of Indo-European roots.]