1 general:: Phrase(s): thin on top Fig. balding. • James is wearing a hat because he’s getting thin on top. • Father got a little thin on top as he got older.
McGrawhill's American Idioms And Phrasal Verbs
2 general:: 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]
American Heritage Idioms