1530s, typical name for an English woman of the lower class, hence "girl, lass, sweetheart," sometimes also "strumpet," from the pet form of Isabel. Often paired with Tom, as Jill was with Jack. Colloquial St. Tibb's Eve (1785) was the evening of the last day, the Day of Judgment, hence "never."
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Isabel
fem. proper name, a form of Elizabeth that seems to have developed in Provence. A popular English name in the Middle Ages; pet forms included Ibb, Libbe, Nibb, Tibb, Bibby, and Ellice. The Spanish form was Isabella, which is attested as a color name ("greyish-yellow") in English from c. 1600; the Isabella who gave her name to it has not been identified, and the usual stories are too late for the date. Related: Isabelline (adj.).