unnatural adj.

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.

c. 1300, naturel, "of one's inborn character; hereditary, innate, by birth or as if by birth;" early 14c. "of the world of nature (especially as opposed to man)," from Old French naturel "of nature, conforming to nature; by birth," and directly from Latin naturalis "by birth, according to nature," from natura "nature" (see nature).

Of events, features, etc., "existing in nature as a result of natural forces" (that is, not caused by accident, human agency, or divine intervention), late 14c. From late 14c. of properties, traits, qualities, "proper, suitable, appropriate to character or constitution;"  from late 15c. as "native, native-born." Also late 15c. as "not miraculous, in conformity with nature," hence "easy, free from affectation" (c. 1600). Of objects or substances, "not artificially cultivated or created, existing in nature" c. 1400. As a euphemism for "illegitimate, bastard" (of children), it is recorded from c. 1400, on the notion of blood kinship (but not legal status).

Natural science, that pertaining to physical nature, is from late 14c.;  natural history meaning more or less the same thing is from 1560s (see history).  Natural law "the expression of right reason or the dictate of religion inhering in nature and man and having ethically binding force as a rule of civil conduct" is from late 14c. Natural order "apparent order in nature" is from 1690s. Natural childbirth is attested by 1898. Natural life, usually in reference to the duration of life, is from mid-15c.; natural death, one without violence or accident, is from mid-15c. To die of natural causes is from 1570s.