thin on top
Becoming bald, as in I notice that he's getting a little thin on top. The top in this euphemism refers to the top of the head. [Second half of 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.