It's naturally assumed that AWOL stormtrooper Finn is simply dubbed as that from birth, but he actually goes by a designated codename, FN-2187, given to him after being snatched from his family and brainwashed (or as the First Order call it, "conditioned") for years. Finn abandons his post after seeing the atrocities the Order commits up close, and rescues an imprisoned Dameron, Dameron decides that the "FN" in his codename means he should be named Finn, and so the character's name is born forever more. Didn't see that one coming, did you?