1520s, "liable to customs or dues;" c. 1600, "according to established usage, habitual," from Medieval Latin custumarius, from Latin consuetudinarius, from consuetitudinem (see custom (n.)). In Middle English it was a noun, "written collection of customs" of a manor or community. Earlier words for "according to established usage" were custumal (c. 1400, from Old French), custumable (c. 1300). Related: Customarily.
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