When something is onrushing, it is coming toward you fast and with force — a train, a wave, a crowd. The picture is of movement that will not stop and is almost on you. People also stretch it to a feeling or a time that seems to rush in all at once, like onrushing panic or onrushing night; that use is more at home in writing than in chat. It is not rare, but it is not an everyday classroom word either: you meet it in news and stories more than at the dinner table. It works as an adjective (an onrushing car). The related noun is onrush: the onrush of the crowd. Onrushing is the -ing form of that picture, used to describe the thing while it is still coming.