That means, literally, “the air/wind that is big.”
And if you remember back a few posts, we had “the small wind.”
The idea that níłch’itsoh conveys is the December timeframe, when storms generally become more aggressive. It is said that around the equinoxes, the summer and winter seasons meet and then turn around to either proceed or walk back. This goes on and results in the rotating seasons.
Try your voice at pronouncing using the clip for níłch’its’ósí.