To lose consciousness; to faint or black out. Used when someone briefly becomes unconscious due to illness, shock, or injury.
暑さで気を失いそうになったので、水を飲んで休んだ。
I almost fainted from the heat, so I drank water and rested.
彼女は血を見て気を失ってしまった。
She lost consciousness when she saw the blood.
事故で頭を打ち、しばらく気を失っていた。
He hit his head in the accident and was unconscious for a while.