sweh₂d-eyo-
“to make one-s own, accustom”custom, costume, consuetude
Iterative of *sweh₂d- giving Latin consuētūdō, English custom, costume, consuetude.
Discussion
The PIE form *sweh₂d-eyo- (to make one's own, to accustom, to establish as customary) is a causative derivative from the reflexive pronoun *swe- (self, one's own), with the verbal extension *-dʰeh₁- (to place) and the iterative *-eyo-. The literal meaning is "to keep placing for oneself" — to establish a pattern, to make something habitual.
Latin suēscere (to become accustomed) and the related consuētūdō (custom, habit — from con-suēscere, "to become accustomed together") gave English: custom (consuētūdinem via Old French), costume (the same word, re-borrowed from Italian — a costume is the customary dress of a place or period), and the archaic suete (accustomed). The word desuetude (the state of having fallen out of custom — dis-suētūdō) preserves the root in legal vocabulary: a law falls into desuetude when it ceases to be customarily enforced.
The connection between "self" and "custom" is conceptually revealing: what is customary is what one has made one's own, what has become part of the self through repetition. Custom IS self-making — the process by which external practices become internal identity.
Greek éthos (ἔθος, "custom, habit") and the related ḗthos (ἦθος, "character, disposition" — the customary behaviour that defines a person) may connect to this root through a different formation, giving English: ethic, ethics, and ethos. The derivation is debated, but the semantic parallel is exact: character IS customary behaviour, and custom IS self-making.
The English word ethnic (from Greek ethnikós, "pertaining to a people/nation" — from éthnos, "people," possibly related to éthos) would then connect national identity to customary practice: an ethnic group is defined by its shared customs, its collective self-making.