Android M is the developer preview of Android Marshmallow.
Android MNC means: Macadamia Nut Cookie, Google's Android OS, after Android L (known as Lollipop). As Android M preview was released on Google I/O 2015, it's preview SDK was released also with Android Studio:
The name has been spotted already in AOSP code and it's used internally at Google.
Obviously, this doesn't mean that Google will necessarily use MNC later this year once it's ready to publicly announce the name it chose for Android M. The company isn't likely to announce the name at Google I/O this week. Instead, Google might simply call it Android M until the final version of it is ready to roll out in fall, just like it did last year with Android L during beta testing.