colloquialism n.

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.