thin on top - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]