An Etymological Dictionary of the German Language/Annotated/Bolle
Bolle (1.), feminine, ‘onion,’ properly identical with the following word; both are subdivisions of a probable primary meaning, ‘bulbaceous.’ It is hardly probable that Greek βολβός, Latin bulbus (whence English bulb), ‘bulb, onion,’ had any influence on the meaning. See also Zwiebel.
Bolle (2.), ‘bulb,’ from Middle High German bolle, Old High German bolla, feminine, ‘bud, bowl’; compare the corresponding Anglo-Saxon bolla, ‘vessel, bowl,’ English bowl (Modern High German Bowle, is borrowed from English). Interesting forms are Old High German hirnibolla, ‘skull,’ and the equivalent Anglo-Saxon heáfodbolla. It is evident that there was originally some such idea as ‘boss-shaped’ in the Old Teutonic word; compare farther Middle High German boln, Old High German bolôn, ‘to roll, throw, hurl.’