Definition -
(1) An incorrect pronunciation.
(2) Using a non-standard word, expression, or pronunciation.

Example -

(1) Incorrectly pronouncing the word mischievous (mis' - chiv - us) as mis - cheev' - ee - us
(2) Always calling Paris Pair - ee after you take an introductory French course.

Etymology -
The word derives via Latin from the Greek barbaros, foreign or strange.
Note: Some dictionaries claim that the word derives from the Greek's way of imitating foreign speech, which to them sounded like bar bar bar (i.e. blah blah blah). However, this can't be correct because the word ultimately derives from the PIE base *barbar (cf. the Sanskrit barbara, which denotes both "stammering" and "non-Aryan").

Oxford English Dictionary -
Its first citation is from 1579:
"Affected with their barbarisme."
(Lyly Euphues (Arb.) 131)

Quotation -
"The grammarians used to distinguish between barbarism, incorrectness in the use of words, and solecism, incorrectness in the construction of sentences."
(Source: Fowler's Modern English Usage s.v. solecism )

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