Foolishness was a quality characterized by having a lack of good sense and judgment and being in opposition to wisdom. Folly was an act of foolishness and a fool was an individual who acted foolish.
The Vulcan soldier Nirak's name became the word for fool in the Vulcan language after a disastrous blunder on his part. Many centuries later Soval warns that Shran name would suffer the same fate if he didn't believe Soval warning about an upcoming attack. (ENT: "Kir'Shara")
In 2254, Vina called Christopher Pike a fool for thinking he could resist the Talosians. Later, after Vina said she got headaches whenever Pike talked strangely, he asked her, "These headaches, they'll be hereditary you know. Would you wish them on a child or a whole group of children?", to which she responded, "Foolish". (TOS: "The Cage")
In 2265, Gary Mitchell called Lee Kelso a fool for having seen the damaged starboard impulse packs and not noticing their condition. Later, when speaking to James T. Kirk, Mitchell joked that, "Compassion and command are a fool's errand." (TOS: "Where No Man Has Gone Before")
In 2266, when he was bluffing his way out of a precarious situation, James T. Kirk said to his counterpart on the Fesarius, "We grow annoyed at your foolishness." (TOS: "The Corbomite Maneuver")