I agree with Dave, with some extensions.
When I have to estimate on something I never did before, I do a bit of research. Even if I’m sitting face-to-face with the client, I ask for some time to look around the Interwebz.
If I can’t find anything similar, I will give a really long timeframe (and probably a huge load of money). If the client asks “Why?”, I break down the problem to small tasks, so they can understand my reasoning better.
However, if I add “because no-one did this before me ever”, the client usually says “we should do it immediately”.