Maybe this is the key to memorable one word lines - set-up and pay-off? Repetition?

Debbie, that's a really good point. Remember that line in Pirates of the Caribbean: Curse of the Black Pearl: "Parlay?" That was used at least three times I can remember: * Elizabeth when she was first captured by th pirates. * Jack when he was about to be whacked by the pirates (he had a hard time remembering the exact word). * One of the pirates toward the very end when they were surrounded by soldiers. That's a classic example of a 'callback' and as you point out, that repetition is one of the reasons a line - even one word - becomes memorable.

I think it also has to do with how Wallace Shawn plays this character (and all his characters). He's so convinced of his own brilliance and cunning, that putting his beliefs in jeopardy is a perfect juxtaposition of his character. This structure always works well for toppling a comic villain (think Syndrome from the Incredibles or the bridge keeper from Monty Python's Search for the Holy Grail). It also helps that the word isn't commonly used, and therefore carries a lot of weight(especially when said repeatedly).

Has anyone suggested Luke Skywalker's "NOOOOOO!" for a one liner? Here's an epic No Montage, that includes Luke and tons of the same line delivered by various characters in LOTR.

Oliver, those are excellent points as well. The word "inconceivable" is a perfect representation of Vizzini's (Wallace) pomposity, thus his continued use of it underscores what an ass-hat he is. Your second point, too: words not commonly used. It's akin to the example I posed from POTC: "Parlay." Playing around with that, we can think of unusual words used by unusual people or in unusual circumstances. Or common words used by unusual people or in unusual circumstances, almost as if the one (or more) words we use are their own FOOW (Fish-Out-Of-Water). For example, a Bishop of some church at a huge cathedral gathering, drops the King's crown as he's about to set it atop the new King and says... What one word would be distinctive in that situation? Here's one that popped into my head: "Balls." Pretty sure that would get a laugh and be a line people would remember. Other suggestions?