Wordorigins.org

Heaven is a word that dates back to the Old English heofon, heben, or one of various forms that appear in extant texts from that age. Its earliest sense is that of the sky, the firmament in which the stars are placed. From Beowulf, line 1571:

swa of hefene hadre scineð rodores candel
(as from heaven, the candle of the sky clearly shines)

The plural form that is commonly used today also dates back this far. From c.825 in the Vespasian Psalter, from Psalms 8:3:

Ic gesie heofenas werc fingra ðinra
(I see the heavens, work of your fingers)

The sense meaning the abode of God, the afterlife, appears a little bit later. From a translation of the gospel of Matthew, c.1000:

Fader ure þu þe eart on heofene
(Our father, you who are in heaven)

The ultimate origin of heaven is not known. It has cognates in Low German, but the Old High German himil, from which the modern German himmel and the Dutch hemel come, the Gothic himins, and the old Norse himinn are not related.

(Source: Oxford English Dictionary, 2nd Edition)