The Collaborative International Dictionary of English v.0.48:

Forceless \Force"less\, a.
   Having little or no force; feeble.
   [1913 Webster]

         These forceless flowers like sturdy trees support me.
                                                  --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

forceless
    adj 1: lacking force; feeble; "a forceless argument" [syn:
           forceless, unforceful] [ant: forceful]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "forceless":
   emasculate, featherweight, feeble, flabby, impotent, inadequate,
   ineffective, ineffectual, inefficacious, invertebrate, lightweight,
   no-account, of no account, powerless, soft, spineless,
   unauthoritative, weak, without any weight