loony [coll.]

Shortened and altered from "lunatic" deriving ultimately from Latin luna, moon, from the once held belief that lunacy or madness depended on the phases of the moon.

We are obviously fascinated by this condition, there are lots of synonyms in English:

absurd, ape, barmy, batty, berserk, bizarre, bonkers, cracked, crazed, cuckoo, daft, delirious, demented, deranged, dingy, dippy, eccentric, erratic, flaky, flipped, flipped out, freaked out, fruity, goofy, half-baked, harebrained, haywire, idiotic, inane, insane, kooky, loony, ludicrous, lunatic, mad, maniacal, mental, moonstruck, nuts, nutty, outrageous, potty, preposterous, psycho, ridiculous, screw loose, screwball, screwy, senseless, silly, touched, unbalanced, unglued, unhinged, unzipped, wacky, whacko, weird, wild.