An Etymological Dictionary of the German Language/hohl
hohl, adj., ‘hollow, concave,’ from the equiv. MidHG. and OHG. hol, adj.; comp. Du. hol, ‘hollow,’ AS. hol, OIc. holr, adj., ‘hollow’; E. hole is an adj. used as a subst., so too AS., OHG., and MidHG. hol, ‘cave.’ The relation of these cognates, which point to Goth. *hula-, ‘hollow,’ to the equiv. AS. holh, E. hollow, has not been explained. The word is usually connected with the root hel (in hehlen), ‘to conceal by covering’; Goth. hulundi, f., lit. the hiding place, ‘cave.’