It's the contraction of morning and afternoon. It's mostly used at work when you start your day, but people on a different timezone are already in their afternoon.
Morternoon! (yes, it's that simple—it's like when you say Morning!)
something you say to greet good friends politely when you meet them from 10:30am to 11:59am
Anson: Good morternoon Martin! How're you doing today? Prepared for the Exco meeting?