c. 1200, custume, "habitual practice," either of an individual or a nation or community, from Old French costume "custom, habit, practice; clothes, dress" (12c., Modern French coutume), from Vulgar Latin *consuetumen, from Latin consuetudinem (nominative consuetudo) "habit, usage, way, practice, tradition, familiarity," from consuetus, past participle of consuescere "accustom," from com-, intensive prefix (see com-), + suescere "become used to, accustom oneself," related to sui, genitive of suus "oneself," from PIE *swe- "oneself" (see idiom).
Custom implies continued volition, the choice to keep doing what one has done; as compared with manner and fashion, it implies a good deal of permanence. [Century Dictionary]
A doublet of costume. An Old English word for it was þeaw. Meaning "the practice of buying goods at some particular place" is from 1590s. Sense of a "regular" toll or tax on goods is early 14c. The native word here is toll (n.).
Custom-house "government office at a point of import and export for the collection of customs" is from late 15c. Customs "area at a seaport, airport, etc., where baggage is examined" is by 1921.