Definitions

1. adjective

being the most eccentric or unreasonable

“He was considered the looniest person in the group with his outlandish ideas.”

2. superlative of ‘loony’

the most insane or mentally unstable

“The looniest among them was locked up in a mental institution.”

3. noun

a person who is considered the most eccentric or unreasonable

“He was known as the looniest person in town for his bizarre behavior.”