Origin and history of i.e.

third person singular present indicative of be, Old English is, from Germanic stem *es- (source also of Old High German, German, Gothic ist, Old Norse es, er), from PIE *es-ti- (source also of Sanskrit asti, Greek esti, Latin est, Lithuanian esti, Old Church Slavonic jesti), third person singular form of root *es- "to be." Old English lost the final -t-.

Until 1500s, pronounced to rhyme with kiss. Dialectal use for all persons (I is) is in Chaucer. Phrase it is what it is, indicating resigned acceptance of an unpleasant but inevitable situation or circumstance about which nothing truly positive can be said, is attested by 2001.

Middle English, from Old English geon "that or those," referring to objects at a distance, "that one (or those) over there," from Proto-Germanic *jaino-, source also of Old Frisian jen, Old Norse enn, Old High German ener, Middle Dutch ghens, German jener, Gothic jains "that, you."

This is reconstructed to be from PIE pronominal stem *i- (source also of Sanskrit ena-, third person pronoun, anena "that;" Latin idem "the same," id "it, that one;" Old Church Slavonic onu "he;" Lithuanian ans "he").

By late 14c. as "farther, more remote" in reference to another. As an adverb from late 15c., a shortening of yonder.

Latin, literally "that is (to say)," from id "that," neuter of is, from PIE pronominal stem *i- (see yon). For est, see is. Usually abbreviated i.e. "to write, or even to say, this in the full instead of in the abbreviated form is now so unusual as to convict one of affectation" [Fowler]. It introduces another way to say something already said, not an example of it (which is e.g.).