I've heard this term so often in my travels in England, in writing as well. I know it means "sassy", but wonder how it came to be used this way.

We Americans, of course, refer to "cheeks" as two specific parts of the body. Or as, in the case of the lower anatomy, our butts.
Here again, the British use the term "bum". To an American, a "bum" is a "shirker of responsibility".

Now I'm even wondering where "sassy" originated.

"Adversity is the whetstone of creativity"