An Etymological Dictionary of the German Language/Floh
Floh, m., ‘flea,’ from MidHG. vlôch, vlô, m., f., OHG. flôh, m.; a common Teut. term; comp. Du. floo, AS. fleáh, E. flea, OIc. fló. It probably means ‘fugitive,’ and is akin to fliehen; hence a Goth. *þláuhs, not *fláuhs, is to be assumed. But even if *fláuhs is the Goth. form, it cannot be connected with either Gr. Ψύλλα or Lat. 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.