Unexpected because Nastasia Philipovna, on her way to Pavlofsk, had thought and considered a good deal, and had expected something different, though perhaps not altogether good, from this interview; but Aglaya had been carried away by her own outburst, just as a rolling stone gathers impetus as it careers downhill, and could not restrain herself in the satisfaction of revenge.
There are no more uses of "impetus" in the book.
Show samples from other sources
Budget cuts were the impetus for the reorganization.
The Supreme Court decision provided the impetus for the change.