prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").
The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.
It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.
1520s as a noun, "a believer in and follower of Christ;" 1550s as an adjective, "professing the Christian religion, received into the Christian church," 16c. forms replacing Middle English Cristen (adjective and noun), from Old English cristen, from a West Germanic borrowing of Church Latin christianus, from Ecclesiastical Greek christianos, from Christos (see Christ). First used in Antioch, according to Acts xi.25-26:
And when he had found him, he brought him unto Antioch. And it came to pass, that a whole year they assembled themselves with the church, and taught much people. And the disciples were called Christians first in Antioch.
Meaning "having the manner and spiritual character proper to a follower of Christ" is from 1590s (continuing a sense in the Middle English word). Christian name, that given at christening, is from 1540s (also continuing a sense from Middle English Cristen). Christian Science as the name of a religious sect is from 1863.