One who is deficient in judgment, sense, or understanding.
One who acts unwisely on a given occasion: I was a fool to have quit my job.
One who has been tricked or made to appear ridiculous; a dupe: They made a fool of me by pretending I had won.
Informal A person with a talent or enthusiasm for a certain activity: a dancing fool; a fool for skiing.
A member of a royal or noble household who provided entertainment, as with jokes or antics; a jester.
One who subverts convention or varies from social conformity in order to reveal spiritual or moral truth: a holy fool.
Archaic A mentally deficient person; an idiot.
Fool me once, shame on you; fool me twice, shame on me. Prov. After being tricked once, one should be wary, so that the person cannot trick you again.
“Fool me once, shame on you. Fool me twice, shame on me. Fool me three times, shame on both of us.” Stephen King, On Writing: A Memoir of the Craft