Where do kids learn to swear? Seriously

Children learn the foul language from all over, but it is the kids' parents who supervise them and teach them how to behave properly.

...or not.

I knew swear words my parents had never heard, before, but I never swore in front of adults or smaller kids. Not after my grandmother washed my mouth out with soap because I let something slip. As an adult, I don't need to swear because my vocabulary is large enough for me to express myself in other ways.