An Etymological Dictionary of the German Language/Annotated/Knabe
Knabe, masculine, ‘boy, lad, youth,’ from Middle High German, late Old High German chnabo, masculine, ‘boy’; also ‘youth, fellow, servant,’ with the originally equivalent variants, Modern High German Knappe, Middle High German knappe, Old High German chnappo (Old High German chnabo and chnappo are related like Nabe and Nappe). Anglo-Saxon cnapa, Old Saxon knapo, and Old Icelandic knape, ‘attendant, squire,’ present some difficulties compared with Anglo-Saxon cnafa, English knave. Equally obscure is the relation of the entire class to the root ken, Aryan gen (Latin genus, gi-gn-o, Greek γένος, γι-γν-ομαι, &c.), with which some etymologists would like to connect it; if it were allied, Old High German chnëht (kn-ëht) also might perhaps be compared.
Knabe may with Old Irish gnia, 'servant,' point to a common base, gnopot, gnepot.