a one-off

one-off

An isolated occurrence; something that happens just once. Oh, that class was a one-off—my interest in French was dead by the time the semester ended. I swear, I don't like him—our kiss was just a one-off! Their performance was just a one-off—they won't be touring.

a ˌone-ˈoff

(British English)
1 a thing that is made or that happens only once and not regularly: This plate is a one-off produced by Minton in 1898. ♢ ‘Are they going to do another concert in the church this year?’ ‘I don’t think so. It was just a one-off.’
2 (informal, humorous) a person who is quite unlike other people: There’ll never be another Charlie Chaplin. He was a one-off.