fool around

"The word has in mod.Eng. a much stronger sense than it had at an earlier period; it has now an implication of insulting contempt which does not in the same degree belong to any of its synonyms, or to the derivative foolish." [OED]

Meaning "jester, court clown" first attested late 14c., though it is not always possible to tell whether the reference is to a professional entertainer or an amusing lunatic on the payroll. As the name of a kind of custard dish, it is attested from 1590s (the food was also called trifle, which may be the source of the name). The verb meaning "to make a fool of" is recorded from 1590s. Related: Fooled; fooling. As an adjective, fool foolish, silly is considered modern U.S. colloquial, but it is attested from early 13c. Feast of Fools (early 14c.), from M.L. festum stultorum) refers to the burlesque festival celebrated in some churches on New Year's Day in medieval times. Fool's gold "iron pyrite" is from 1882. Fool's paradise "state of illusory happiness" is from mid-15c. Fool around is 1875 in the sense of "pass time idly," 1970s in sense of "have sexual adventures." Foolosopher, a most useful insult, turns up in a 1549 translation of Erasmus. Fools ballocks is described in OED as an old name for the green-winged orchid.