Origins
The English word 'sun' belongs to one of the oldest and most securely reconstructable word families in the Indo-European tradition. It descends from Old English 'sunne,' from Proto-Germanic *sunnōn, ultimately from the PIE word for the sun, reconstructed as *suh₂en- (also written *sh₂wen-). The PIE sun-word appears across virtually every branch of the language family: Latin 'sōl,' Greek 'hēlios' (from earlier *hāwelios), Sanskrit 'sūrya' and 'svar,' Avestan 'hvar,' Welsh 'haul,' Lithuanian 'sáulė,' Old Church Slavonic 'slŭnĭce,' Albanian 'diell' (from *dhyēl, a reshaped form), and Gothic 'sauil.' The breadth and regularity of these cognates make the PIE sun-word one of the cornerstones of comparative linguistics.
What makes the reconstruction particularly interesting is that PIE had two different stem formations for the sun: an l-stem *sóh₂wl̥ (which produced Latin 'sōl,' Gothic 'sauil,' and Welsh 'haul') and an n-stem *suh₂en- (which produced the Germanic *sunnōn line, Lithuanian 'sáulė,' and Sanskrit 'sūrya'). These are not separate words but two inflectional forms of the same word — PIE had a complex noun paradigm in which the same word could appear with different stem consonants in different grammatical cases. Over time, individual daughter languages generalized one stem or the other. English inherited the n-stem through Germanic; the l-stem entered English later through Latin borrowings like 'solar,' 'solstice,' and 'solarium.'
The deeper etymology connects *suh₂en- to the PIE root *sewH-, meaning 'to give birth.' Under this analysis, the sun was conceived by Proto-Indo-European speakers as 'the one who gives birth' — to light, warmth, the day, and by extension, life itself. This metaphor has parallels across mythologies: in Norse mythology, the sun goddess is called Sól; in Hindu tradition, Sūrya is the solar deity; the Greek Hēlios drives the sun-chariot across the sky. The deification of the sun is among the most universal features of human religion.
Proto-Indo-European Roots
In Proto-Germanic, the sun was grammatically feminine — *sunnōn was a feminine noun, and this gender persists in German ('die Sonne'), Dutch ('de zon'), and the Scandinavian languages. Old English 'sunne' was likewise feminine. The Norse mythology of the female sun goddess Sól reflects this grammatical gender. English lost grammatical gender during the Middle English period, so 'sun' is now neuter ('it'), but the historical femininity of the sun in Germanic cultures stands in contrast to the Mediterranean tradition, where the sun (Latin 'sōl,' Greek 'hēlios') was masculine and the moon feminine.
The word 'Sunday' — Old English 'Sunnandæg,' literally 'day of the Sun' — is a loan-translation (calque) of Latin 'diēs Sōlis,' which was itself a translation of Greek 'hēméra Hēlíou' (day of the Sun-god Helios). The seven-day planetary week spread across the Roman Empire, and Germanic-speaking peoples translated the Roman planet-names into their own equivalents. Sunday preserved the sun-reference, while other days mapped Roman gods onto Germanic ones: Tuesday (Tiw/Mars), Wednesday (Woden/Mercury), Thursday (Thor/Jupiter), Friday (Frīg/Venus).
The compound 'solstice' (from Latin 'sōlstitium,' literally 'sun-standing') refers to the moments when the sun appears to stand still at its northernmost or southernmost point before reversing direction. 'Solar' (from Latin 'sōlāris') entered English in the fifteenth century. 'Parasol' (from Italian 'parasole,' literally 'shield against the sun') arrived in the seventeenth century. All of these Latin-channel words carry the l-stem form of the PIE sun-word, while native English 'sun,' 'sunny,' 'sunshine,' 'sunlight,' 'sunrise,' and 'sunset' carry the n-stem form. English speakers thus use both ancient variants of the PIE sun-word daily, usually without any awareness that 'sun' and 'solar' are different forms of the same six-thousand-year-old word.