From 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]