Definition of open up in US English:

open up

phrasal verb

  • Begin shooting.

    ‘the enemy artillery had opened up’
    • ‘US tanks and artillery opened up, flattening houses where insurgents were thought to be hiding.’
    • ‘Men who went over-the-top in trenches stood little chance when the enemy opened up with their machine guns.’
    • ‘When their entreaties were ignored, an infantry charge was ordered, the artillery opening up behind it.’
    • ‘At the same time, American artillery opened up on other, less sensitive rebel strong points.’