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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.