‘Melon’ comes from the ancient Greek word for apple and other seed-containing fruits.  The Greeks called a melon a ‘melopepon’ (‘apple-gourd’), which became shortened to ‘melon’ or similar in many languages.  In Tuscany, where prosciutto with melon is a classic dish, the fruit still goes by its ancient name, ‘popone’.  A similar word is used in Romanian.  

Melons are relatives of cucumbers, squashes and gourds.  Perhaps bizarrely, a cantaloupe is more closely related to a gherkin than it is to a watermelon...