1640s, "gourd-like fruit, of a deep orange-yellow color when ripe, of a coarse decumbent vine native to North America," an alteration of pompone, pumpion "melon, pumpkin" (1540s), from French pompon, from Latin peponem (nominative pepo) "melon," from Greek pepon "melon." The Greek word is probably originally "ripe," on the notion of "cooked (by the sun)," from peptein "to cook" (from PIE root *pekw- "to cook, ripen"). With ending conformed to words in -kin.

Figuratively, in 19c. (and later) U.S. vernacular, it has meant both "stupid, self-important person" and "person or matter of importance" (as in some pumpkins).

Pumpkin-pie is recorded from 1650s. Pumpkin-head, American English colloquial for "person with hair cut short all around" is recorded by 1781. Vulgar American English alternative spelling punkin attested by 1806.

America's a dandy place:

The people are all brothers:

And when one's got a punkin pye,

He shares it with the others.

[from "A Song for the Fourth of July, 1806," in The Port Folio, Philadelphia, Aug. 30, 1806]

late 14c., meloun, "herbaceous, succulent trailing annual plant," or its sweet, edible fruit, from Old French melon (13c.) and directly from Medieval Latin melonem (nominative melo), from Latin melopeponem, a kind of pumpkin, from Greek mēlopepon "gourd-apple" (name for several kinds of gourds bearing sweet fruit), from mēlon "apple" (see malic) + pepon, a kind of gourd, which is probably a noun use of pepon "ripe" (see pumpkin).

Among the earliest plants to be domesticated. In Greek, melon was used in a generic way for all foreign fruits (compare similar use of apple). The Greek plural of "melon" was used from ancient times for "a girl's breasts."

Proto-Indo-European root meaning "to cook, ripen." 

It forms all or part of: apricot; biscuit; charcuterie; concoct; concoction; cook; cuisine; culinary; decoct; decoction; drupe; dyspepsia; dyspeptic; eupeptic; kiln; kitchen; peptic; peptide; peptone; precocious; pumpkin; ricotta; terra-cotta.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pakvah "cooked, ripe;" Avestan -paka- "cooked;" Greek peptein "to cook, ripen, digest," pepon "ripe;" Latin coquere "to cook, prepare food, ripen, digest, turn over in the mind," Oscan popina "kitchen;" Lithuanian kepti "to bake, roast;" Old Church Slavonic pecenu "roasted;" Welsh poeth "cooked, baked, hot."

also jack-o-lantern, jack-a-lantern, jackolantern, 1660s, "night-watchman;" 1670s as a local name for a will-o-the-wisp (Latin ignis fatuus), mainly attested in East Anglia but also in southwestern England. Literally "Jack of (with) the lantern;" see Jack + lantern. The extension to carved pumpkin lanterns is attested by 1834 in American English.