"반사" - 반 (反): reflect; send back; oppose ("반대", "반응", "반문", ...) - 사 (射): shoot, illuminate ("사격", "발사") - 반사: reflect. So you probably said happy new year to him/her first, and he is saying the same thing back to you. He just used the word "reflect back" to make it funny. 반사 is not commonly used this way though. It will sound childish and rude in most situations.