A solecism is a mistake in grammar or a breach in etiquette, according to my old and well-loved red paperback Merriam-Webster dictionary. This is one of those words I've definitely heard but did not actually know the proper meaning of until I looked at it on dictionary.reference.com. Thank you, Word of the Day!
What I find most interesting about this word is its origins. The word solecism comes from the Latin soloecismus, meaning 'to speak incorrectly'. The Greek word soloikos, from which the Latin is derived, means 'an inhabitant of Soloi.' Soloi was a city in ancient Greece, in Cilicia, in modern-day Turkey. Athenians regarded the dialect spoken by those in Soloi to be unsophisticated, a corrupt form of Attic Greek.
There's an interesting chart of usage of this word over at Wordnik: it seems to have enjoyed wide usage in the 1800s, but is rarely seen today.