loon:
(plural: ‘loons‘; pronunciation: the letters ‘oo’ are said with a long ‘o’ sound as in the word “loo” — for the right pronunciation, please click here)
a foolish or lazy person;
a person who is not right in his head (deranged/crazy);
a simple minded person, not necessarily deranged/crazy,

a loon?
My granddaughter thinks that I’m a loon for trying to make her understand things she doesn’t understand.
Our political science professor used to tell us that there are more loons in politics than there are in any other system, and the people who don’t think politicians are the best loons are loons themselves.
The manager is fed up with the loons in his staff and grumbles all the time that it’s impossible to run the office with these under-qualified, inefficient loons!
In addition, the word ‘looney’, ‘loony’ or ‘luny’ is more commonly used to refer to a crazy, deranged or simple minded person; and “looney bin” is a slang/informal word for a hospital for mentally ill persons. For an article that explains the words ‘loony’, ‘lunatic’, etc., please click here.
