Lose one's reserve, temper, or self-restraint; do or say something out of keeping with one's position or character. For example, A teacher should never forget herself and shout at the class. Shakespeare used it in Richard II (3:2): “I had forgot myself: am I not king?” [Late 1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use forget oneself in a sentence
To save oneself one must first lose oneself—forget oneself, when occasion arises—for the sake of other people.Austin and His Friends | Frederic H. Balfour
The true first love is only that man or woman who can cause one to forget oneself.When Dreams Come True | Ritter Brown
It was agreeable news, but it was nothing to make one forget oneself.The Whirligig of Time | Wayland Wells Williams
All these candidates forget that the surest manner to reach the White House is not to think of it—to forget oneself and to act.Diary from March 4, 1861, to November 12, 1862 | Adam Gurowski
To forget oneself in greater interests is to escape from a prison.The Outline of History: Being a Plain History of Life and Mankind | Herbert George Wells