Brain Acid

Most of modern YouTube.

Any information that makes you feel like you got stupider simply by taking it in. The sheer idiocy of this content may be enough to completely Ruin Your Day. Brain Acid can also make somebody very angry instead of making them feel dumber, possibly to the point where they claim it's extremely stupid. In fiction, there are UnTrashes that can be considered Brain Acid, such as Big Red Button, Hip Wit' Da Kidz, and everyone's favorite, All Just A Dream.

Alternatively, the reverse can be true. Brain Acid can be information that's so smart, it makes somebody feel like they're gonna spurt or that their brain will melt. Bonus Points if it really does melt. This can then only qualify as true Brain Acid if they encounter a Brain Error that breaks them mentally.