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.
orthodox adj.
mid-15c., in reference to theological opinions or faith, "what is regarded as true or correct," from Late Latin orthodoxus, from Greek orthodoxos "having the right opinion," from orthos "right, true, straight" (see ortho-) + doxa "opinion, praise" (from dokein "to seem;" from PIE root *dek- "to take, accept").
Of other subjects than religion or theology from 1640s. Meaning "of or pertaining to the Greek Church," by 1772. As a noun, "member of the Eastern Orthodox Church, from 1580s. In reference to a branch of Judaism, recorded from 1853; as a noun meaning "an Orthodox Jew," by 1889.