unseemly (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. 1200, semeli, semlich, of persons, "of pleasing or good appearance, handsome, fair," also, of conduct, "proper, tasteful, decorous; good for a purpose," and generally, "pleasant, suitable, fitting," probably from Old Norse soemiligr "becoming, honorable," from soemr "fitting" (see seem).

The notion is "suited to the object, occasion, purpose, or character." From late 14c. as "worthy of respect, honorable." Related: Seemliness. A  document from 1440 has seemlity. Old Norse had also soemleitr "fine to look at."

c. 1200, impersonal, hit semeth (it seems), "it appears (that something is so);" also with adjectives or phrases, "to appear to be (in some condition), have or present an appearance of being," from Old Norse soema "to honor; to put up with; to conform to (the world, etc.)," a verb derived from the adjective soemr "fitting."

This is reconstructed to be 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" (also compare same).

With other verbs (seem to be, etc.) from c. 1200. Sense of "appear to oneself, think oneself" is from 1630s. Also in Middle English "to present oneself, appear; be visible, be apparent" (late 14c.), hence, of a fact, etc., "be evident, apparent, or obvious." The sense of "be fitting or appropriate, be expedient" (c. 1300) is the etymological one, but it is obsolete except in derived seemly, unseemly. Related: Seemed; seeming.