Meanings will depend a lot on context. ie Often, there is more than one possible meaning and the conversation that comes before will determine what is exactly meant. Here's one set of possibilities: “Hang on “ Don't leave yet. Wait for me, I'll be there soon. “Slipped my mind” Sorry, I completely forgot. “CU then” I'll see you (at that time/place being discussed) “Bye for now” Goodbye - see you later. “I’m back” I was away. Now you can contact me again.

“Hang on“ = "Wait" “Slipped my mind” = "I knew that, but I can't remember now." “CU then” = "I will see you at the time we agreed on." “Bye for now” = "Goodbye now, but I will see you soon." “I’m back” = "I wasn't here or available, but now I'm here." By the way, it is "instant message", not "massage" which is the action of pressing and rubbing someone's body with your hands.