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in one go; in one gulp; in one breath; without stopping; without pausing; without a rest; in one sitting; at a stretch
suddenly; all of a sudden; all at once
水を一気に飲み干した。
I drank all the water in one gulp.
階段を一気に駆け上がった。
I ran up the stairs in one go.
気温が一気に下がった。
The temperature dropped sharply all at once.