This etymology parsing the word as a compound defines grapes by reference to the process used to produce wine, trampling the grapes. The interesting thing about it is that a noun referring to a natural object is etymologized by reference to a cultural product: the nature of grapes is to be trampled in order to produce wine. The fundamental part of the etymology is obviously the second syllable, [tru], which suggested truō. Then it was necessary to find a word which could account for the first syllable: as far as grapes are concerned, extracting the juice is done by trampling, which suggested that the first element was the name of the foot. That implies a secondary formal manipulation (change of [p] to [b]). The Excerpta e codice Darmstadino 2773 of the same Etymologicum have a different formulation, in the form of an elliptic etymology: βότρυς· περὶ τὸ ποσὶ τρίβεσθαι· οἱονεὶ, πότρυς· καὶ τροπῆ τοῦ π εἰς β, where the assumed etymon, truō, is not explicit but is replaced by a modern synonym, tribō "to wear out, to thresh, to rub"