A former French coin of little value; now used to mean a tiny amount of money or something worthless.
From French 'sou', derived from Latin 'solidus', a Roman gold coin. The French coin was worth very little by the 18th century, leading to its metaphorical use for anything trivial.
A sou was so worthless that the French phrase 'not worth a sou' (pas un sou) became their version of 'not worth a red cent'! The Roman 'solidus' coin was originally valuable, but inflation over centuries reduced the sou to pocket change—a reminder that money's worth is always relative.
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