Old English heofon "home of God," earlier "sky, firmament," probably from Proto-Germanic *hibin-, dissimilated from *himin- (cf. Low German heben, Old Norse himinn, Gothic himins, Old Frisian himul, Dutch hemel, German Himmel "heaven, sky"), perhaps from PIE root *kem-/*kam- "to cover" (cf. chemise). [Watkins derives it elaborately from PIE *ak- "sharp" via *akman- "stone, sharp stone," then "stony vault of heaven"].
Plural use in sense of "sky" is probably from Ptolemaic theory of space composed of many spheres, but it also formerly was used in the same sense as the singular in Biblical language, as a translation of Hebrew plural shamayim. Heaven-sent (adj.) attested from 1640s.