doing something in one breath / in one go; quickly and all at once (often used as 一気に).
疲れていたのでコーヒーを一気に飲んだ。
I was tired, so I drank the coffee in one go.
宿題を一気に片付けて、夜はゆっくり休んだ。
I finished the homework all at once and rested leisurely at night.
彼女は一気に階段を駆け上がった。
She ran up the stairs in one go.
an interjection used as a party cheer meaning 'chug!' or 'drink up!' (often chanted repeatedly).
乾杯のあと、みんなで『一気！一気！』と叫んでビールを飲んだ。
After the toast, everyone shouted 'Ikki! Ikki!' and chugged their beer.
司会者が合図をして『一気！』の掛け声で全員がグラスを空にした。
The host signaled and, with the shout 'Ikki!', everyone emptied their glasses.