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.

Source: Webster's Revised Unabridged Dictionary (1913)