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.
afraid adj.
"impressed with fear, fearful," early 14c., originally the past participle of the now-obsolete Middle English verb afray "frighten," from Anglo-French afrayer, Old French affrai, effrei, esfrei "disturbance, fright," from esfreer (v.) "to worry, concern, trouble, disturb," from Vulgar Latin *exfridare, a hybrid word meaning literally "to take out of peace."
The first element is from Latin ex "out of" (see ex-). The second is Frankish *frithu "peace," from Proto-Germanic *frithuz "peace, consideration, forbearance" (source also of Old Saxon frithu, Old English friu, Old High German fridu "peace, truce," German Freide "peace"), from a suffixed form of PIE root *pri- "to be friendly, to love."
A rare case of an English adjective that never stands before a noun. Because it was used in the King James Bible, it acquired independent standing and thrived while affray faded, and it chased off the once more common afeared. Colloquial sense in I'm afraid "I regret to say, I suspect" (without implication of fear, as a polite introduction to a correction, admission, etc.) is recorded by 1590s.
Her blue affrayed eyes wide open shone [Keats, "The Eve of St. Agnes," 1820]