I've never heard the term "earworm" before this thread. It's having a song stuck in your head, I'm gathering. Canadian word/phrase?
I don't
think it's that much more common a usage in Canada, but the number of (US) people in this thread indicating unfamiliarity with it made me wonder.
It seems it's an Anglicization of a German expression (other sources confirm the etymology, so I'm not just relying on Wiki here).
I know my (USian) BF is familiar with it; it's come up in convo and I didn't have to explain it - but it may be more region-specific in the US than it is up here (Canadian linguistic regionality is generally more subtle and less distinct).
Sunflower