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Old English dell "dell, hollow, dale" (perhaps lost and then borrowed in Middle English from cognate Middle Dutch/Middle Low German delle), from Proto-Germanic *daljo (cf. German Delle "dent, depression," Gothic ib-dalja "slope of a mountain"); related to dale (q.v.).
rogue's cant 16c.-17c. for "a young girl of the vagrant class," of uncertain origin.
A Dell is a yonge wenche, able for generation, and not yet knowen or broken by the vpright man. ... [W]hen they have beene lyen with all by the vpright man then they be Doxes, and no Dells. [Thomas Harman, "A Caveat or Warning for Common Cursitors," 1567]