Pepon

The word pumpkin originates from the word pepon (πέπων), which is Greek for "large melon", something round and large. 
The French adapted this word to pompon, which the British changed to pumpion and to the later American colonists became known as pumpkin.

Pumpkins were easy enough to come by and good for carving and got absorbed both into the carved lantern tradition and the associated prank. 

Over time, kids refined the prank and began carving crude faces into the pumpkins to kick up the fright factor and make the lanterns look like disembodied heads. By the mid-1800s, Stingy Jack’s nickname was applied to the prank pumpkin lanterns that echoed his own lamp, and the pumpkin jack-o’-lantern got its name.

Toward the end of the 19th century, jack-o’-lanterns went from just a trick to a standard seasonal decoration, including at a high-profile 1892 Halloween party hosted by the mayor of Atlanta. In one of the earliest instances of the jack-o’-lantern as Halloween decor, the mayor’s wife had several pumpkins—lit from within and carved with faces—placed around the party, ending Jack O’Lantern’s days of wandering, and beginning his yearly reign over America’s windowsills and front porches.