from Wiktionary, Creative Commons Attribution/Share-Alike License
- proper n. A certain mutant strain of fruit fly (Drosophila melanogaster) with heightened sensitivity to ethyl alcohol (ethanol), causing it to lose postural control at lower concentrations of the drug.
Coined by Monica S. Moore et al. in 1998 (see 1998 quotation), from “cheap date”, the idea being that it is less expensive to buy an evening's worth of drinks for a person with a lower tolerance for alcohol. (Fruit-fly mutants are frequently given such clever or whimsical names.) (Wiktionary)