The Collaborative International Dictionary of English v.0.48:

Groundless \Ground"less\, a. [AS. grundle['a]s bottomless.]
   Without ground or foundation; wanting cause or reason for
   support; not authorized; false; baseless; as, groundless
   fear; a groundless report, rumor, or assertion. --
   Ground"less*ly, adv. -- Ground"less*ness, n.