是

Unclear. Possibly taken from an oracle-bone form of 智 (OC *treh), composed of 子, 止 and 冊.

“This [an independent pronoun] > this, it [object recapitulation], be right, correct, so > to be, indeed”. The modern copulative sense only emerged by the Eastern Han dynasty (Dobson, 1964; Norman, 1988; Zürcher, 2013), and gradually replaced the archaic 唯 (OC *ɢʷi, “to be”) and the classical copulative construction with 也 (OC *laːlʔ).

This was due to the lack of a left-branching copula as archaic 唯 shifted into a contrasting and restricting particle, as well as its frequent use in presentational copulative constructions, often in the form of "A, 是 B 也" ("A, this is B"), which led to it being gradually reinterpreted as the copula instead of 也. Also note the semantic opposition with literary negative copula 非 (OC *pɯl) as in "right" and "wrong", which emerged by the Warring States period from the frequent collocation of the expression "是 X 也, 非 Y 也" (This is X, not Y) (Pulleyblank, 1995) .

To understand this development, compare 之 (OC *tjɯ, “him, it, this > possessive”), 其 (OC *kɯ, “he, his, its > dialectal possessive”). Compare French c'est which in colloquial speech often replaces copula est.

From Proto-Sino-Tibetan *m-daj ~ m-di (“that; this”). In Chinese, cognate with 寔 (OC *djɯɡ, “this (subject contrastive)”), 實 (OC *ɦliɡ, “this (subject contrastive)”), 之 (OC *tjɯ, “this; him; her; it (oblique)”), 諟 (OC *djeʔ, “to examine, to consider”). In Tibeto-Burman, cognate with Tibetan དེ (de, “that”), Jingpho dai, ndai (“this”), and Burmese ဒီ (di, “this”).