Originally the rhyme, "Fi fi foe fum, I smell the blood of an English man" was "Fi fi foe fum, I smell the blood of a Christian man" and was changed for controversial reasons. It's part of old Norwegian lore, something to do with the trolls being worshiped by the people in pagan times and then forgotten about when many turned to Christ. Naturally, they were peeved and had to go into hiding, otherwise they would've all been killed.