

General relativity - my nerdy finger tattoo
The explanation:
It's the Einstein field equations, the basis of the general relativity. It basically says that the geometry of spacetime tells matter how to move and in return matter/energy curves spacetime.
Thanks to a good friend I've gotten into theoretical physics lately after being in a bit of an intellectual slump for a few years, so this is my personal reminder that I'm smarter than I thought.
Also, yes, I know finger tattoos like that are prone to fading, but that's alright with me. I knew that going in, and it's a reminder I need right now. If and when it fades, it has done its job.