from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Happening, done, or made only once.
- n. Something that is not repeated or reproduced.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Occurring once; one-time.
- n. Something that is done, created, etc. only once, and often quickly, simply, or improvisationally.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a happening that occurs only once and is not repeated
Probably from foundry work, for which making reusable molds is expensive and expediency the rule for molds not to be reused. (Wiktionary)