An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
open up on (someone or something)
to attack someone or something, to fire a gun or other weapon at someone or something
The man with the gun opened up on the people on the bus.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a group of words, or in other words, a phrase that has a meaning different from the literal meaning of the words in it.