"avoidance of words deemed unlucky," 1885, from Latinized form of Greek kledon "omen or presage contained in a word or sound," also "report, rumor, tidings; fame" (from PIE root *kele- (2) "to shout") + -ism.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit usakala "cock," literally "dawn-calling;" Latin calare "to announce solemnly, call out," clamare "to cry out, shout, proclaim;" Middle Irish cailech "cock;" Greek kalein "to call," kelados "noise," kledon "report, fame;" Old High German halan "to call;" Old English hlowan "to low, make a noise like a cow;" Lithuanian kalba "language."
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
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<a href="https://www.etymonline.com/word/cledonism">Etymology of cledonism by etymonline</a>
Harper, D. (n.d.). Etymology of cledonism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/cledonism