A top line representing the level above a man with outstretched arms (大 (dà)). Ancient representation of the character show a man with a large head, an anthropomorphic representation of heaven.
Original meaning of the word is "sky". Such concept is difficult to depict with a pictogram, hence the current form. Influenced by the glyph, the character was frequently glossed in Old Chinese documents with 顛 (“crown of the head”), and such had led to incorrect hypotheses which regarded 天 as a derivation of 顛 or 巔, "top (of the head/hill)".
This character was previously reconstructed to have a *tʰ-initial in Old Chinese, but this hypothesis has been largely discounted in recent reconstructions, in light of evidence from early Chinese accounts of dialectal pronunciations, transcription of other languages, as well as cognate/derived characters within Chinese, which has all pointed to a voiceless resonant initial *l̥- in Old Chinese. The book Shiming showed that, by c. 200 CE, this initial had already produced dialectal variation in the pronunciation: People in central-western China pronounced it with the back of the tongue, like 顯[*xˤenʔ], while people in eastern China pronounced it with the tip of the tongue, like 坦[*tʰˤanʔ]. An old northwestern dialect variant survives as 祆 (xiān, “God of the Zoroastrians”) [xen], and the word 天竺 (Tiānzhú, “India”) was used to transcribe Old Persian *Hind-uka ("India").
Further etymology is unknown. While it was reconstructed with a /*tʰ-/ initial, it was frequently compared with tengri, the name for God in early Turkic and Mongolic peoples' languages. This now appears unlikely. The only certain external cognate is the Central Bai (a language closely related to Chinese) word heinl (“sky, heaven”)[xẽ⁵⁵].
From Old Japanese. Usually appears as a bound form only appearing in compounds. Compare the formation of 目 (ma, bound form in compounds) and 目 (me, standalone form), 手 (ta, bound form in compounds) and 手 (te, standalone form).