fire on

fire (up)on (someone or something)

To aim a weapon at someone or something and discharge it. Hey, we're not the enemy—quit firing on us!
See also: fire

fire (up)on someone or something

to shoot at someone or something with a weapon; to shoot in the direction of someone or something. (Upon is formal and less commonly used than on.) The troops fired upon the advancing army. The cops fired on Max.
See also: fire, on