Accepting the fact that being lean (<=15% body fat) is optimal for male health and attractiveness. A person who believes this is said to be lean pilled.
This belief exists to counter the narrative pushed by powerlifters and perma bulkers, which states that one should just get as big as possible and prioritize lifting numbers above everything else.
Friend: What happened to you? You used to be so strong and swole!
Me: I decided that feeling and looking my best is better than being able to bench 4 plates. I took the lean pill.