Congratulate oneself, boast, as in He plumed himself on his victory. This idiom transfers the bird's habit of dressing its feathers to human self-satisfaction. [First half of 1600s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.