take it on/upon yourself to do something
take it on (oneself) to (do something)
To decide to do something even though it was not one's responsibility. I took it on myself to print out these information packets for the meeting since I knew the data might be confusing. The kids took it on themselves to wash your car—I'm so proud of them for doing it without being told!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
take it on/upon yourself to do somethingdecide to do something without asking anyone for permission: He took it upon himself to dismiss my secretary, which he had no right to do.
Farlex Partner Idioms Dictionary © Farlex 2017