Detailed:
N - Mission, short for Narnia mission, reffering to the movie The Chronicles of Narnia and attatching the word mission displays that the mission (trip) is very long like the trip it took the kids from the movie traveling from the ice land to Narnia.

General Definition: a very long trip from the starting point to the destination.

Adj. Something that is expected to have a long lifespan by virtue of having existed for a long time, based on the lindy effect, named for the New York restaurant Lindy's, coined by Albert Goldman and popularized by Nassim Taleb.