vent one's spleen

1. Express one's anger, as in Some people see town council meetings as a place where they can vent their spleen. This expression uses vent in the sense of “air,” and spleen in the sense of “anger,” alluding to the fact that this organ was once thought to be the seat of ill humor and melancholy. [First half of 1600s]

Example Sentences

It is much harder to sustain vitriol through the laborious process of a handwritten letter than it is to vent one’s spleen with a few characters that can be read by millions in a nanosecond.

From Washington Post

To spoil for fights and vent one's spleen, as Simmonds does so regularly, isn't criticism, but trolling.

From The Guardian