singing/humming the "Jaws" theme while sneaking up or following someone, consists of going "duh nah, duh nah, duhnah....duhnah nah nah nah!!" and is often ended very dramatically. is used to ether frighten or amuse.
you see your friend stocking shelves at work, so you sneak behind him while going ""duh nah, duh nah, duhnah....duhnah nah nah nah!!" getting louder the closer you get. That is what "Jawsing" is