'Custom' is Latin for 'making one's own' — from 'suescere' (to accustom), rooted in PIE *se- (self).
A traditional and widely accepted way of behaving or doing something specific to a particular society, place, or time; also, the habitual patronage of a shop or business; also (in plural), duties levied on imports.
From Anglo-Norman 'custume,' from Old French 'coustume' (custom, habit, practice), from Vulgar Latin *cōnsuētūmen, an alteration of Latin 'cōnsuētūdō' (habit, usage, custom), from 'cōnsuēscere' (to accustom, to become used to), composed of 'con-' (intensive) and 'suēscere' (to become accustomed), from PIE *swē-dh- (to make one's own), related to *s(w)e- (self). The word thus literally means 'a making one's own' — a practice so habitual it has become part of oneself. Key roots: cōnsuētūdō (Latin: "habit, custom, usage"), suēscere (Latin: "to become accustomed"), *s(w)e- (Proto-Indo-European: "self, one's own").
English 'custom' and 'costume' are doublets — both come from the same Latin word 'cōnsuētūdō' (habit). French split them: 'coutume' kept the meaning 'habitual practice,' while Italian 'costume' specialized as 'manner of dress' (because clothing was the most visible custom of a people). English borrowed both.