'watermelon' is an English compound of 'water' + 'melon'. 'Water' comes from Old English 'wæter', and 'melon' comes from Old French 'melon' (from Latin 'melo' or 'melopepo').

'melon' entered Middle English from Old French 'melon', which in turn came from Latin 'melo' and ultimately from Greek 'mēlon' meaning 'apple' or 'tree fruit'. The compound 'watermelon' developed in English to describe melons with very watery flesh and was used from the early modern period to refer specifically to Citrullus lanatus.

Originally formed simply as a compound meaning a 'melon with a lot of water' or 'watery melon'; over time it came to denote the specific large, juicy fruit (and the species) known today.