Smarmy ('smär · mē) is an adjective meaning sincere-acting or earnest, but in an obviously false, irritating way. It can also have overtones of flattery. In order to be smarmy, you need to be not just complimenting somebody, but licking boots and looking really pleased with yourself. An affectation.
According to the American Heritage Dictionary, smarmy has its origins in the verb "to smarm," itself derived from "to smalm," which means "to smear." Other than the fact that it comes from some dialect, I haven't been able to find where the word arose.
I find it interesting that "unctuous" is usually given as an apt synonym for smarmy. Unctuous means "greasy," from Latin ungueo, to smear with grease. So both words come from a sense that a falsely sincere person seems greasy, or is smearing grease over their speech.
Also see smarm, the sort of speech that a smarmy person affects, as well as the person (him/her)self.
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