Solecism

sol·e·cism (n.): 1. A nonstandard usage or grammatical construction; 2. A violation of etiquette; 3. An impropriety, mistake, or incongruity.

Latin soloecismus, from Greek soloikismos, from soloikizein, to speak incorrectly, from soloikos, speaking incorrectly, after Soloi (Soli), an Athenian colony in Cilicia where a dialect regarded as substandard was spoken.

(from the American Heritage Dictionary)