1590s, from un- (1) "not" + reachable (see reach (v.)).
Entries linking to unreachable
un- 1
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.
reach v.
Middle English rēchen, from Old English ræcan, reccan "to reach out, stretch or extend outward, hold forth, extend in continuity or scope," also "to succeed in touching, succeed in striking;" also "to address, speak to," also "to offer, present, give, grant."
This is proposed to be from Proto-West Germanic *raikejanan "stretch out the hand" (source also of Old Frisian reka "to give, pay," Middle Dutch reken, reiken, Old High German reihhen, reichen "give, reach out, get," Dutch reiken, German reichen "to reach, to pass, to hand, to give; to be sufficient"), from Proto-Germanic *raikijanau, which is probably from PIE root *reig- "to stretch, stretch out, be stretched; be stiff."
Sometimes 16c. spelled retch. As "to hand (someone something), give" from c. 1300. The meaning "arrive at, succeed in getting to" is early 14c.; that of "succeed in influencing" is from 1660s. Related: Reached; reaching. Shakespeare uses the now-obsolete past tense form raught (Old English ræhte).
Colloquial reach-me-down "ready-made" (of clothes) is recorded from 1862, from notion of being on the rack in a finished state.