When a person who originally is unattractive ages/hits puberty and is transformed into a sexy beast.
Originates from Pokemon, when a weak Magikarp evolves and becomes a powerful Gyarados.
Jane: Did you hear about Brian moving back to town?
Karen: Yeah! I never imagined he'd go through puberty so well and get so HOT!
Jane: I know! The Gyarados Affect worked so well on him!