E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The Mahometans say that the eating of a melon produces a thousand good works. So named from Melos.
Etre un melon. To be stupid or dull of comprehension. The melon-pumpkin or squash is soft and without heart, hence être un melon is to be as soft as a squash. So also avoir un cur de melon (or de citrouille) means to have no heart at all. Tertullian says of Marcion, the heresiarch, he has a pumpkin [peponem] in the place of a heart [cordis loco]. It will be remembered that Thersits, the railer, calls the Greeks pumpkins (pepons).