in a rut
In a settled or established habit or course of action, especially a boring one. For example, We go to the seashore every summer—we're in a rut, or After ten years at the same job she says she's in a rut. This expression alludes to having a wheel stuck in a groove in the road. [Early 1800s]
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.