I too am surprised that "one-off" is considered slang. 'Round these parts it is not considered slang, it is a perfectly reputable word!

From the OED:
b. one off: a single example of a manufactured product; something not repeated; a prototype. Freq. (with hyphen) attrib. or as adj. Also transf. and fig. Cf. off adv. 13, once-off adj. s.v. once adv. B. 7b.

The first use was:
1934 Proc. Inst. Brit. Foundrymen XXVI. 552 A splendid one-off pattern can be swept up in very little time.


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