In northern Germany, as well as Luxembourg and South Denmark, people greet each other with “Moin!” (or “Mojn!). One possible origin comes from the German good morning (guten Morgen -> Morgen -> Moin). However, “Moin” is often not only used in the morning, but in the afternoon and evening as well. Another attempt to explain the origin of “Moin” points at the Low German word “moi” (meaning beautiful, good). The issue is further complicated by the use of “Moin Moin”, where the first “Moin” might stand for good and the second “Moin” stand for morning.
– David Huesmann
 K?ppers’ W?rterbuch der deutschen Umgangssprache, Klett, 1997