One Breath, One Go, One Sitting
adverb
It's weird to think of having one energy. But, what gives you energy? You do it one time every couple of seconds. That would be taking breaths. So, "one energy" is one breath.
いっき
The reading is the same as the ones you learned with the individual kanji, though いち from 一 is shortened to いっ.
父はテーブルの上のチョコレートを一気にたべた。
My father ate the chocolates that were on the table all in one sitting.
たくさんの人がいたので、デザートは一気になくなった。
There were so many people, so the dessert was gone at once.
人気ドラマのシーズンを丸ごと一気にみてしまった。
I binge-watched a whole season of the popular drama in one sitting.