The Collaborative International Dictionary of English v.0.48:

pantywaist \pantywaist\ n.
   A timid man or boy considered childish or unassertive.

   Syn: sissy, pansy, milksop, Milquetoast.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

pantywaist
    n 1: a timid man or boy considered childish or unassertive [syn:
         sissy, pantywaist, pansy, milksop, Milquetoast]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "pantywaist":
   Lord Fauntleroy, Milquetoast, Percy, baby, big baby, chicken,
   cream puff, crybaby, doormat, dull tool, effeminate, goody-goody,
   gutless wonder, invertebrate, jellyfish, lightweight, lily,
   meek soul, milksop, mollycoddle, namby-pamby, nance, nancy,
   nebbish, nonentity, old woman, pansy, pushover, sad sack, sissy,
   softling, softy, sop, weak sister, weakling, wishy-washy