Webster's Revised Unabridged Dictionary · 1913

Sou

Sou , noun

[French sou, Old French sol, from Latin solidus a gold coin, in Late Latin, a coin of less value. See Sold, n., Solid, and and compare Sol, Soldo.]

An old French copper coin, equivalent in value to, and now displaced by, the five-centime piece (f of a franc), which is popularly called a sou.