An Etymological Dictionary of the German Language/Füllen
Füllen, n., from the equiv. MidHG. vülîn, OHG. fulîn, n., besides MidHG. vüle, OHG. fulî, n., ‘foal’; for the affix -în-, denoting the young of animals, see under Schwein. Based upon Fohlen (Goth fula); hence *ful-ein, n. has to be assumed in Goth.; comp. MidLG. völen, Du. veulen, OIc. fyl. Another derivative of ful- is OHG. fulihha, MidHG. vülhe, f., ‘filly,’ pointing to Goth. *fuliki.