WordNet (r) 3.0 (2006):

sissy
    adj 1: having unsuitable feminine qualities [syn: effeminate,
           emasculate, epicene, cissy, sissified,
           sissyish, sissy]
    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:

130 Moby Thesaurus words for "sissy":
   Lord Fauntleroy, Milquetoast, Percy, afraid, aunt, auntie, baby,
   big baby, blood brother, brethren, brother, bub, bubba, bud, buddy,
   chicken, chicken liver, chickenhearted, country cousin, cousin,
   cousin once removed, cousin twice removed, coward, cowardly, cowed,
   cream puff, crybaby, daughter, daunted, dismayed, doormat,
   dull tool, effeminate, epicene, fainthearted, father, fearful,
   first cousin, foster brother, fraid-cat, fraidy-cat, frater, funk,
   funker, funking, funky, goody-goody, grandnephew, grandniece,
   granduncle, great-aunt, great-uncle, gutless wonder, half brother,
   henhearted, intimidated, invertebrate, jellyfish, kid brother,
   lightweight, lily, lily liver, lily-livered, meek soul,
   milk-livered, milksop, milksoppish, milksoppy, mollycoddle, mother,
   mouse, mousy, muliebrous, namby-pamby, nance, nancy, nebbish,
   nephew, niece, nonentity, nuncle, nunks, nunky, old woman,
   old-womanish, overtimid, overtimorous, panic-prone, panicky, pansy,
   pantywaist, pigeonhearted, prissy, pushover, rabbity, sad sack,
   scaredy-cat, second cousin, sis, sissified, sissyish, sister,
   sister-german, sistern, soft, softling, softy, son, sop,
   stepbrother, stepsister, timid, timorous, unc, uncle, uncs,
   unmanly, unmanned, uterine brother, weak, weak sister, weak-kneed,
   weakhearted, weakling, wet, white feather, white liver,
   white-livered, womanish, yellow