"La Belle Dame Sans Merci" isn't the most obvious title in the world for an English poem, because it's not in English. It's in French and, as those of you in French 1 already figured out, it translates to "The beautiful lady without mercy." But why is the title in French? Why couldn't Keats just title it, "The beautiful lady with a heart of stone"?
Well, as you may have already guessed, the title is an allusion to a much earlier work of literature. It's from a medieval romance by the French poet Alain Chartier. The poem itself has many of the same elements as a medieval romance (knights, fair ladies, fairies, dream sequences…), so by titling the poem with a line from a famous romance, Keats calls up all those associations right from the beginning.