The prefix "ap" is short for "apo" because the h replaces the o. The prefix in Greek means "away; off; or appart", but the prefix un English generally means detached, separate, or derivative. The jungle tribe is detached or separate from the rest of the world. The rest of the word, "helios" could possibly refer to the ancient Greek god that represents the sun. In Greek mythology the Greek gods "helios" and "apollo" were combined into a figure called "Sol". Not sure how this would be related to aurelion sol though. All together this word could mean "away or apart from the sun" which i guess they cant see the sun from the trees in the jungle. Probably wrong but this is just what I found in the dictionary.