不旋踵
bù​xuán​zhǒng

in a split second

不旋踵 bù​xuán​zhǒng
in a split second
(literary) in the brief moment it takes to turn one's heel; very quickly; in a very short time
without retreating
(literary) to not turn one's heel back; to fight to the death without retreating; courageous and undaunted