What's the "een" in Halloween?
early 13c., "penitential chastisement; punishment," from Old French descepline (11c.) "discipline, physical punishment; teaching; suffering; martyrdom," and directly from Latin disciplina "instruction given, teaching, learning, knowledge," also "object of instruction, knowledge, science, military discipline," from discipulus (see disciple (n.)).
Sense of "treatment that corrects or punishes" is from notion of "order necessary for instruction." The Latin word is glossed in Old English by þeodscipe. Meaning "branch of instruction or education" is first recorded late 14c. Meaning "military training" is from late 15c.; that of "orderly conduct as a result of training" is from c.1500.
c.1300; see discipline (n.). Related: Disciplined; disciplines; disciplining.