c. 1300, "visually unpleasing, unattractive, ugly;" mid-14c., "unfitting, indecent, inappropriate," from un- (1) "not" + seemly (adj.). Similar formation in Old Norse usoemiligr. Related: Unseemliness.
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. 1200, semlich, "of pleasing or good appearance," also "proper, tasteful, decorous; good for a purpose," from Old Norse soemiligr "becoming, honorable," from soemr (see seem). Related: Seemliness. Old Norse also had soemleitr "fine to look at."
late 12c., "to be fitting, be appropriate, be suitable;" c. 1200, "to appear to be, have or present the appearance of being;" from Old Norse soema "to honor; to put up with; to conform to (the world, etc.)," verb derived from adjective soemr "fitting," from Proto-Germanic *somiz (source also of Old English som "agreement, reconciliation," seman "to conciliate," source of Middle English semen "to settle a dispute," literally "to make one;" Old Danish söme "to be proper or seemly"), from PIE *somi-, suffixed form of root *sem- (1) "one; as one, together with."
The sense of "be fitting, be appropriate" in English is the etymological one, but it is obsolete except in derived seemly, unseemly. Related: Seemed; seeming.
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Definitions of unseemly from WordNet
not in keeping with accepted standards of what is right or proper in polite society;