The Collaborative International Dictionary of English v.0.48:

Sou \Sou\, n.; pl. Sousor. [F. sou, OF. sol, from L. solidus a
   gold coin, in LL., a coin of less value. See Sold, n.,
   Solid, and and cf. Sol, Soldo.]
   An old French copper coin, equivalent in value to, and now
   displaced by, the five-centime piece (1/20 of a franc), which
   is popularly called a sou.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sou
    n 1: a former French coin of low denomination; often used of any
         small amount of money; "he hasn't a sou to his name"

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "sou":
   Deutschmark, Mark, Reichsmark, a continental, a curse, a damn,
   a darn, a hoot, afghani, anna, bagatelle, baht, bauble, bean,
   bibelot, bit, brass farthing, button, cent, centavo, centime,
   conto, curio, dollar, dong, farce, farthing, feather, fig,
   fleabite, florin, folderol, franc, fribble, frippery, gaud, gewgaw,
   gimcrack, guilder, gulden, hair, halfpenny, hill of beans, jest,
   joke, kickshaw, kip, knickknack, knickknackery, kopeck, krona,
   krone, lira, milreis, minikin, mockery, molehill, peppercorn,
   peseta, picayune, pie, piece of eight, pin, pinch of snuff,
   pinprick, pistareen, pound, rand, rap, red cent, rial, row of pins,
   ruble, rupee, rush, shekel, shilling, shit, snap, sneeshing, sol,
   stiver, straw, toy, trifle, trinket, triviality, tuppence,
   two cents, twopence, whim-wham, won, yen