Out of one's thoughts or inclinations. It is often put as get something out of one's system, as in You need to get your ex-husband out of your system, or At the annual all-chocolate buffet I try everything, which gets it out of my system for at least a month, or Let him complain as much as he wants so he'll get it out of his system. This idiom uses system in the sense of “all one's physical and mental functions.” [c. 1900]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.