An Etymological Dictionary of the German Language/Annotated/geil
geil, adjective, ‘rank, wanton, obscene, lewd,’ from Middle High German and Old High German geil, ‘of savage strength, wanton, exuberant, merry, joyous’; for the change of meaning on the transition from Middle High German to Modern High German compare Schimpf. The primary meaning. ‘unrestrained, joyous,’ follows from Gothic gailjan, ‘to rejoice’; compare Old Saxon gêl, Dutch geil, Anglo-Saxon gâl. To the Teutonic cognates Lithuanian gailùs, ‘passionate, furious, sharp, painful, sympathetic,’ and gailěti-s, ‘to injure’; Old Slovenian zělu (from gailo), ‘violent,’ adverb zėlo, ‘very’ In the compound Biebergeil appears the Middle High German noun geil, geile, ‘testicle.’