Fum, Fum, Fum

So we sang this one caroling yesterday afternoon.  I'd heard it a time or two, but didn't know much about it.  I thought it was pronounced with a short u (rhymes with bum), but it's from Spain (Catalonia) originally.

Fum appears to refer to smoke, like from a chimney (think winter cottage scene).  For a familiar related word, think of no smoking signs in Spanish - no FUMar. 

Or it might mean "strum," or to sound like a strum. 

Either way, the song is 200-300 years old.