1offpowdercoating.com
One-off as an adjective means 'done, occurring, or made only once', and as a noun (much less common) 'something that occurs, is done, or is made only once'. Depending on context, it can be interpreted as meaning 'unique' or 'exceptional', as you suggest; good synonyms are "singular," "unique,"or "one-shot."
The word stems from the British use of off in commerce to indicate a quantity of items produced at one time: "Please supply 500 off." A one-off, then, was an item produced only once, and the current usage is a figurative application of this technical sense.
One-off is chiefly a Briticism, first found in the 1930s; it has been been increasingly common in America in recent years.
Courtesy of Randomhouse
http://www.randomhouse.com
I'm not in this to do hundreds of the same thing, so I chose the name 1offpowdercoating