on sight
As soon as someone or something is or can be seen. Police have orders to shoot the suspect on sight.
on sight
Also, at sight. Immediately upon seeing, as in The soldiers threatened to shoot looters on sight, or He's able to multiply those three-digit figures at sight. [Second half of 1400s]
do something on ˈsight
do or feel something as soon as you see somebody/something: She complained constantly about the hotel, which she hated on sight.The soldiers were ordered to shoot on sight. on sight
Immediately upon being seen: threatened to shoot looters on sight.