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mid-14c., "morally weak," from Old French fraile "weak, frail, sickly, infirm" (Modern French frêle), from Latin fragilis "easily broken" (see fragility). Sense of "liable to break" is first recorded in English late 14c. The U.S. slang noun meaning "a woman" is attested from 1908.
A woman, esp a young woman: in persuading frails to divulge what they know (1905+)