An Etymological Dictionary of the German Language/Annotated/Floh
Floh, masculine, ‘flea,’ from Middle High German vlôch, vlô, masculine, feminine, Old High German flôh, masculine; a common Teutonic term; compare Dutch floo, Anglo-Saxon fleáh, English flea, Old Icelandic fló. It probably means ‘fugitive,’ and is akin to fliehen; hence a Gothic *þláuhs, not *fláuhs, is to be assumed. But even if *fláuhs is the Gothic form, it cannot be connected with either Greek Ψύλλα or Latin pulex, since neither vowels nor consonants are in accord. Fliegen too is unrelated, since the final sound of its stem is g only, and not h.