means A small, usually round dose of medicine meant to be swallowed whole — and by extension, anything (or anyone) hard to take.

from From Latin 'pilula,' a 'little ball,' the diminutive of 'pila,' meaning ball. The word rolled through Old French and into Middle English still tiny and round, and English kept both senses: the literal medicinal pellet, and the figurative 'tough pill to swallow,' where the swallowing is metaphor and the bitterness is real.