early 15c. "causing bodily or mental discomfort, affording no comfort," from un- (1) "not" + comfortable (adj.). Intransitive meaning "feeling discomfort, ill-at-ease" is attested from 1796. Related: Uncomfortably.
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.
mid-14c., "affording mental or spiritual comfort," from Anglo-French and Old French confortable "comforting; pleasant, agreeable," from conforter "to comfort, solace" (see comfort (v.)); also see -able. Meaning "cheering, cheerful" is from c. 1400. Meaning "offering physical comfort" is attested from 1769; that of "in a state of tranquil enjoyment" is from 1770.
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Definitions of uncomfortable from WordNet
conducive to or feeling mental discomfort;
the uncomfortable truth
grew uncomfortable beneath his appraising eye
this kind of life can prove disruptive and uncomfortable
an uncomfortable way of surprising me just when I felt surest
the teacher's presence at the conference made the child very uncomfortable