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"