From Middle High German knobelouch, with dissimilation from Old High German chlobalouh. The latter part of the word is, of course, Lauch (“leek”). The first part originally pertains to klieben (“to cleave”), but was later adapted to the large group of terms for thick objects beginning with kn- (cf. Knopf, Knoten, Knolle, etc.). The same in Dutch knoflook.
- Knoblauch in Duden online