A loon is either a certain species of bird or a person who is crazy, illogical, mentally ill, or just foolish. The second definition came as a shortened abbreviation for the word lunatic.
A loonie is a Canadian coin worth one Canadian dollar. It has an image of a loon on it. When the currency was introduced in the 1980s, the nickname for the coin quickly came in use.
Loony or looney is an adjective that describes something or someone as being outside of their right mind, crazy, or lacking the ability to make logical decisions. Something can be loonier or looniest.
As a noun loony is a person who is illogical or insane. The plural is spelled loonies.
The noun form for being loony is looniness.
The term loony bin is for a mental illness hospital where people are institutionalized because they cannot safely live on their own. This term is highly derogatory and should be avoided.
Loony tunes is a synonym for loony. Usually people go loony tunes or are loony tunes. It comes from a popular American children’s cartoon series of the same name where the characters always acted illogically. Note that loony toon is a misspelling, but looney tunes is an acceptable alternate spelling.
While the loonie rose against the yen and the greenback, it held steady against the aussie and the euro. [London South East]
Milne is harder to stereotype – she is clearly neither a loony nor someone who you can imagine wrapping their arms around a tree. [The Guardian]