one-off

From one +‎ off. Probably from foundry work, for which making reusable molds is expensive and expediency the rule for molds not to be used only once.

A term long used by artists who are printmakers and sculptors to indicate a unique print or casting. If using traditional edition numbering, a one-off would be a "1/1", which is said, "one of one" meaning the first print in an edition of one.

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one-off (comparative more one-off, superlative most one-off)

1. Occurring once, independent of any pattern; one-time.

   The government insisted that the embarrassing loss of the tax records was a one-off event.

   Seeing Halley's Comet is a one-off, once-in-a-lifetime opportunity.

   • 1905, Foundry, volume 50, page 198:

     If such a casting was wanted in a hurry — a one-off job — there would be no question of molding it on a machine.

   • 2006 September 1, EIU Country Analysis, Malta: Country outlook:

     Moreover, given that recent measures to cut the deficit have been more one-off than of a structural fiscal nature, meeting this debt target is likely only

   • 2015 May 6, Colin O'Carroll, “Fermanagh woman thought registering a royal baby was a one-off honour... now Princess Charlotte has come along she's done it twice”, in Belfast Telegraph‎, retrieved 2015-05-14:

     A County Fermanagh woman has officiated at the formal registration of the birth of Princess Charlotte at Kensington Palace. It's the second time she's been on royal baby duty at the palace as she previously officiated at the registration of Prince George. She said at the time that it was a "one-off" honour, but that has now been disproved with a repeat visit for Princess Charlotte.

   • 2020 December 16, “Network News: "Robust case" for Fawley branch line reopening”, in Rail, page 14:

     In July, South Western Railway and Network Rail ran a one-off Class 159 to Fawley, to demonstrate the line's viability. [...] A second 'fact-finding' trial passenger train for DfT officials is now proposed.

2. Singular; unique; special; remarkable.
   • 2015 May 13, Hunter Skipworth, “BMW to create one-off tribute to 3.0 CSL”, in Evo‎, retrieved 2015-05-14:

     BMW is to create a one-off tribute to the iconic 3.0 CSL.

• Much more common outside US. Relative usage frequency 5% (COCA versus BNC).

• nonce
• one-shot

one-off (plural one-offs)

1. (idiomatic) Something that is done, created, etc. only once, and often quickly, simply, or improvisationally.

   I'll put together a quick one-off as a sample so we can taste the recipe.

2. Something unique and remarkable.

   It is a one-off; there is nothing else like it.

   • 2017, David Walliams [pseudonym; David Edward Williams], Bad Dad, London: HarperCollins Children’s Books, →ISBN:

     Frank couldn’t believe that his father had put Queenie back together and not told him. More secrets. More lies. The boy guessed that the coat of yellow paint was to disguise her. Queenie was a one-off. A Mini with a Union Jack painted on her would be a dead giveaway to the police.

• break the mold, break the mould

• “one-off”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.