set in one's ways, be
Be inflexible, fixed in one's habits, as in She's too set in her ways to go out and buy a dog. This idiom uses set in the sense of “in a rigid position,” a usage dating from about 1300.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.