on duty

late 13c., from Anglo-French duete, from Old French deu "due, owed; proper, just," from Vulgar Latin *debutus, from Latin debitus, past participle of debere "to owe" (see debt). Related: Duties. The sense of "tax or fee on imports, exports, etc." is from late 15c.; duty-free as a noun is attested from 1958.