From Middle English loppe; the verb is a back-formation from the noun.
From Middle English loppe ("flea, spider"), from Old English loppe ("spider, silk-worm, flea"), from Proto-Germanic *luppǭ ("flea, sandflea", originally, "jumper"), from Proto-Germanic *luppijaną ("to jump, dart"). Cognate with Danish loppe ("flea"), Swedish loppa ("flea"). Compare also Middle High German lüpfen, lupfen (“to raise”, obsolete also “to rise”).