a circle inscribed within a triangle.
Origin of incircle
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
see go around in circles.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.