Old English heofon "home of God," earlier "the visible sky, firmament," probably from Proto-Germanic *hibin-, a dissimilation of *himin- (source also of Low German heben, Old Norse himinn, Gothic himins, Old Frisian himul, Dutch hemel, German Himmel "heaven, sky"), which is of uncertain and disputed origin.
Perhaps it means literally "a covering," from a PIE root *kem- "to cover" (which also has been proposed as the source of chemise). Watkins derives it elaborately from PIE *ak- "sharp" via *akman- "stone, sharp stone," then "stony vault of heaven."
The English word is attested from late 14c. as "a heavenly place; a state of bliss." The plural use in sense of "sky" probably is from the Ptolemaic theory of space as composed of many spheres, but it also formerly was used in the same sense in the singular in Biblical language, as a translation of Hebrew plural shamayim. Heaven-sent (adj.) is attested from 1640s.