discipline
Before "discipline" meant self-restraint, it meant being taught. Latin disciplina, from discipulus (pupil) — a discipline was the body of instruction a learner received. Same root as disciple, doctrine, docent, decent. The harsh sense (chastisement, military discipline) is downstream; underneath, discipline is reception, not imposition. Self-discipline, in the older register, is self-teaching.
The modern word names a hard thing — self-restraint, military obedience, the rod for correction. By the time we reach for it in adult life, we are already braced. The older word didn't brace. Disciplina in Latin meant teaching, instruction, learning — the body of what a discipulus received. A discipulus was a pupil; not a person doing hard things, but a person to whom things were being shown. The discipline lived on the receiving side of the relation.
The cousin is right there in the word: disciple and discipline are the same Latin root. To take on a discipline is to enter the receiving. Underneath sits PIE *dek-, "to take, to accept, to receive" — the same root as doctrine, doctor, decent, dogma. The whole cluster is about reception. The harsh sense (Old French monastic descipline: both the rule you live under and the scourge you mortified the flesh with) is a late layer. Self-discipline, in the older register, is self-teaching.
- disciple — discipline's near twin; one who receives teaching
- doctrine — what is taught; the body of received instruction
- doctor — one who teaches; agent-noun from docere, "to teach"
- document — originally a teaching example; later, a record
- docent, docile — the teaching one and the teachable one; both sides of the relation
- decent, decorum — fitting to receive; what is properly accepted
- dignity — worthy of being received with regard
- dogma — what is held; the received belief
- orthodox, paradox, heterodox — rightly-held, against-held, otherwise-held