To take the bulk pill, or to be bulk pilled, means to go all in while bulking. Your focus is entirely on eating as much as possible to gain as much muscle mass as possible. Someone who is bulk pilled will disregard aesthetics and even health in favour of sheer bulk. Many power lifters are bulk pilled.
Derived from red pill/ blue pill
Guy 1: Fuck cardio and macros, I'm just tryna get big!
Guy 2: Based and bulk pilled.