Where Did It Come From?
One afternoon while chatting, one of my sisters mentioned that, the previous week, after a long and strenuous day she had flopped on the couch and said, "Whew- I've had the biscuit!" Now, anyone in our family- actually, anyone from the Maritimes- would know right away what she meant. Her husband however, who was raised in Ontario, stared at her like she had suddenly started speaking a foreign language. He had no idea what the idiom meant, and actually didn't believe it was one until she googled "had the biscuit" and proved it.
As to it being a uniquely Canadian expression, well as I said, I hadn't realized that, but every site online I checked said the same thing- that the idiom originated in Canada and was pretty much limited to here. Who knew?
In any case, that is the origin of the idiom "had the biscuit" and if you'd never heard it before, well, feel free to use it. I see no reason for such a great phrase to be limited solely to Canadian use.