Definition & Meaning of "loony"

She came up with a loony idea to build a roller coaster in her backyard.

His loony behavior at the party embarrassed everyone who was there.

The teacher dismissed his loony explanation for why he did n't do his homework.

He 's a complete loony for trying to ride his bike down that steep hill.

They all called him a loony for talking about traveling to the moon.

She 's a loony for believing in all those wild conspiracy theories.