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.
1590s, "being," from Late Latin entitatem (nominative entitas), from ens (genitive entis) "a thing," proposed by Caesar as present participle of esse "be" (see is), to render Greek philosophical term to on "that which is" (from neuter of present participle of einai "to be," from PIE root *es- "to be"). Originally abstract; concrete sense in English is from 1620s.
i.e. means that is to say, & introduces another way (more comprehensible to the hearer, driving home the speaker's point better, or otherwise preferable) of putting what has been already said; it does not introduce an example, & when substituted for e.g. in that function ... is a blunder. [Fowler]
masc. proper name, from French, from Latin Isidorus, from Greek Isidoros, literally "gift of Isis," from Isis (see Isis) + dōron "gift" (from PIE root *do- "to give"). St. Isidore, archbishop of Seville (600-636) wrote important historical, etymological, and ecclesiastical works and in 2001 was named patron saint of computers, computer users, and the internet. Related: Isidorian.