What's that children's book with a giant, a father and son and a banjo?

I remember this book from the 80's when I was a little kid, but it had a father and son who were outcasts in a village. The village was terrorized by a large giant/monster, who would come in and eat people or livestock or something. The father and son went in to town, played a song on a banjo, and got the giant to fall down after dancing. Then they became heroes. They kept saying a really weird phrase, and I can't for the life of me remember the phrase. I remember the illustrations though, with the dark shadowy monster eating an entire cow. What was the name of this book?!