fool

fool a fool and his money are soon parted proverbial saying, late 16th century.
a fool at forty is a fool indeed proverbial saying, early 16th century, from the poet Edward Young (1683–1765), and meaning that someone who has not learned wisdom by the age of forty will never learn it. A mid 16th-century sermon has the same idea in reference to the age of forty-one, ‘And he that hath not learned some experience or practice and trade of the world by that age will never be wise.’
a fool may give a wise man counsel sometimes used as a warning against overconfidence in one's judgement; saying recorded from the mid 14th century.
fool me once, shame on you; fool me twice, shame on me if someone is deceived twice their own stupidity is to blame; saying recorded from the early 17th century (in the earliest versions, the verb is deceive).
fool's errand a task or activity that has no hope of success.
fool's gold a brassy yellow mineral that can be mistaken for gold, especially pyrite.
fool's paradise a state of happiness based on a person's not knowing about or denying the existence of potential trouble.
there's no fool like an old fool often used to suggest that the folly of an older person is particularly worthy of castigation. The saying is recorded from the mid 16th century.

see also fools, a man who is his own lawyer has a fool for a client, more people know Tom Fool than Tom Fool knows.