"the way something bends" (coastline, mountain range, etc.), 1777, earlier "round bend of a stream" (1620s), from trend (v.); sense of "general course or direction" is from 1884. Sense of "a prevailing new tendency in popular fashion or culture" is from c. 1950.
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
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Harper, D. (n.d.). Etymology of trendy. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/trendy