MAEN means Morning, Afternoon, Evening, Night. It's an invented word that is helpful when you're talking (chatting or writing) to people in different parts of the world or who may read a note at different times.
For instance, when you have a meeting on a conference line with people in New Zealand and India, and some are currently "in" the afternoon, while others are "in" the morning, they can greet each other with "good maen!"
It's pronounced may-'n.
The word was invented by Robin Lampert prior in 2004 (probably around 2000, but I'll have to check).