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