The word heaven may share an ancient root with hammer and stone
In Beowulf, the Old English heofon simply meant the sky. Linguists trace it back to a Proto-Germanic form, and one theory links that to an Indo-European word for stone, perhaps the stone vault overhead. If so, heaven's cousins include the Sanskrit for thunderbolt, a Greek word for anvil, and the English hammer.
By around the year 1000, heofon had taken on its Christian sense of the place where God lives, and by 1159 it appears in Middle English as heven. Its relatives run across the Germanic languages, from Gothic himins to modern German Himmel and Dutch hemel. Beyond that the trail becomes uncertain. One proposal ties it to a root meaning to cover or shroud; another to a word for stone that shows up as Persian asman, meaning both sling-stone and sky, and Sanskrit asman, rock and firmament.
The stone idea fits how early peoples pictured the sky. Mesopotamians imagined three, or sometimes seven, domes over a flat Earth, each made of a different precious stone. The lowest, of jasper, held the stars; the outermost was personified as An, the sky god. Venus was the goddess Inanna, the Sun her brother Utu and the Moon their father Nanna. Heaven belonged to the gods alone. When Gilgamesh asks who can climb to heaven, the answer is that only the gods live there forever, while every human soul, good or bad, went down to a dim underworld. Temples served as the link; the Ekur at Nippur was called the mooring rope between heaven and earth.
The Hebrew Bible shares much of this picture. Its word for heaven, shamayim, is plural, and phrases like heaven of heavens suggest layers. Prophets glimpse God's council but learn nothing of what heaven looks like, and the dead rest in Sheol rather than rising upward. Only Enoch, who was taken by God, and Elijah, carried off in a chariot of fire, are possible exceptions, and the texts leave both ambiguous.
Change came during the Second Temple era, under Persian, Greek and Roman rule. Greek thought contributed the soul's immortality, and the idea spread that souls came from the divine realm and longed to return. Zoroastrian teachings on heaven, hell and judgement may also have shaped the Abrahamic faiths. In the Gospels, Matthew mostly swaps Mark's and Luke's Kingdom of God for Kingdom of Heaven, probably because it suited his audience better.
Source: Heaven
