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.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).