A notional substance which is used to remove squicky ideas and pernicious memes from one's mind, or occasionally as prophylaxis against such memes becoming established. Unfortunately, it doesn't actually exist.

I have it on somewhat good authority, though, that both ethanol and THC are somewhat effective. Apparently, if you get drunk or stoned soon enough after being exposed to an icky idea, basically before your brain can complete the next write transaction, you just might not have to remember it. Alas, this doesn't work for scrubbing away old, painful memories. Opinions vary on whether this is a good thing or not.