Entries related to unflattering
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.
"Of somewhat doubtful etymology" [OED]. Liberman calls it "one of many imitative verbs beginning with fl- and denoting unsteady or light, repeated movement" (for example flicker, flutter). If it is related to flat the notion could be either "caress with the flat of the hand, stroke, pet," or "throw oneself flat on the ground" (in fawning adoration). The -er ending is unusual for an English verb from French; perhaps it is by influence of shimmer, flicker, etc., or from flattery.
Meaning "give a pleasing but false impression to" is from late 14c. Sense of "show (something) to best advantage" is from 1580s, originally of portraits. Related: Flattered; flattering.
Others are reading
an unflattering portrait