"To prime" is to fill, load, prepare for use, etc.
You prime a pump by putting some water in the pipes from the well.
You prime your muzzleloading rifle by putting black powder in the flash pan or by placing a cap on the nipple.
You prime your mind for a hard exercise by starting with a simple exercise.
I assume you can prime your thirst for a beer by having a small drink before you start with the big drinks.