A sou is a very small amount of money, like a penny or a cent.
The word sou makes people think of old times or of not having much money.
Examples:
I found a sou on the ground.
She doesn't have a sou to her name.
He gave me a sou for my birthday.
I won't give you a sou for that old toy!
I have not a sou in my pocket.
I saved every sou I had to buy a new toy.