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.