Homer uses ἀτρύγετος as an epithet of the sea and sky. The etymology has traditionally been taken to be α participle formed from ἀ + τρυγάω, unharvested or barren, but Beekes says this is "formally not easy" and discusses a couple of other possibilities. What is it that doesn't work about this etymology?
If from τρυγάω, the expected form would be ἀτρύγητος, since in such formations the stem vowel is lengthened (and long α becomes η in Ionic). Chantraine tentatively mentions the possibility that ἀτρύγετος might be a shortening of this expected form metri causa, but that would be an irregular development.