The Collaborative International Dictionary of English v.0.48:

Milquetoast \Milquetoast\ n. [From Caspar Milquetoast, a
   character in a cartoon strip by H. T. Webster, The Timid Soul
   (1935).]
   a timid, unassertive man or boy fearful of confrontation and
   easily manipulated and dominated.

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

WordNet (r) 3.0 (2006):

Milquetoast
    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:

42 Moby Thesaurus words for "Milquetoast":
   baby, big baby, chicken, chicken liver, coward, crybaby, doormat,
   dull tool, fence-sitter, fraid-cat, fraidy-cat, funk, funker,
   gutless wonder, invertebrate, jellyfish, lightweight, lily liver,
   meek soul, milksop, mollycoddle, mouse, mugwump, namby-pamby,
   nebbish, nonentity, pansy, pantywaist, pushover, sad sack,
   scaredy-cat, shilly-shally, sissy, softling, softy, sop, waverer,
   weak sister, weakling, white feather, white liver, wobbler