1810, "a colloquial word or phrase," one peculiar to the language of common conversation, from colloquial + -ism. Meaning "colloquial quality or style" is from 1818. Sometimes conversationism (1853) was used.
1751, "pertaining to conversation," from colloquy "a conversation" + -al (1). From 1752 as "peculiar or appropriate to the language of common speech or familiar conversation," especially as distinguished from elegant or formal speech. Related: Colloquially.
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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Harper, D. (n.d.). Etymology of colloquialism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/colloquialism