The RHHDAS has "melonhead" from 1932.

This--a large antedate from my file-cleaning--is from the Oxford

Gazette (N.Y. state), 25 September 1818, pg. 2, col. 2:

The profound editor of the Advocate has deigned to notice us as a

"devotee" of MR. CLINTON and the State Government, and bestows upon us a few

stale and hacknied epithets, such as "small fry," "melon head," &c. which are

peculiarly characteristic of his intellectual acumen, deep research,

brilliant and inventive imagination. (...) What may we next expect when we

find in the train of his auxiliaries the evil genius of Kinderhook _melon_

patches,* a man well drenched in crime, and his superior in every thing but

native perfidy and hollow hearted sycophany, and in that his equal. (...)

* Thereby hangs a _Tale_