Vulgar

It is probably used most often to mean in a sexual way, but it can mean any coarse or crude act, e.g., "Smacking gum in public at a social event looks vulgar."

What do you mean by 'smacking'?

"Smacking" gum, means to chew gum vigorously with the mouth partially open so that an audible sound is made. I just used it as an example of vulgar behavior that has nothing to do with ***, nudity, or revealing clothing.

Is this expression 'smacking gum' used only in the US or in Canada as well? Also I guess it would be correct if I say to someone 'stop smacking your gum'

?

It is used in the U.S. I don't know whether or not it is used in Canada or Britain. And, yes, one could say (and mothers have for as long as gum has been around,) "Stop smacking your gum"! Mothers usually follow that with, "Now, spit it out!"