The Collaborative International Dictionary of English v.0.48:

Custom \Cus"tom\ (k[u^]s"t[u^]m), n. [OF. custume, costume,
   Anglo-Norman coustome, F. coutume, fr. (assumed) LL.
   consuetumen custom, habit, fr. L. consuetudo, -dinis, fr.
   consuescere to accustom, verb inchoative fr. consuere to be
   accustomed; con- + suere to be accustomed, prob. originally,
   to make one's own, fr. the root of suus one's own; akin to E.
   so, adv. Cf. Consuetude, Costume.]
   [1913 Webster]
   1. Frequent repetition of the same act; way of acting common
      to many; ordinary manner; habitual practice; usage; method
      of doing or living.
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            And teach customs which are not lawful. --Acts xvi.
                                                  21.
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            Moved beyond his custom, Gama said.   --Tennyson.
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            A custom
            More honored in the breach than the observance.
                                                  --Shak.
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   2. Habitual buying of goods; practice of frequenting, as a
      shop, manufactory, etc., for making purchases or giving
      orders; business support.
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            Let him have your custom, but not your votes.
                                                  --Addison.
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   3. (Law) Long-established practice, considered as unwritten
      law, and resting for authority on long consent; usage. See
      Usage, and Prescription.
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   Note: Usage is a fact. Custom is a law. There can be no
         custom without usage, though there may be usage without
         custom. --Wharton.
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   4. Familiar aquaintance; familiarity. [Obs.]
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            Age can not wither her, nor custom stale
            Her infinite variety.                 --Shak.
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   Custom of merchants, a system or code of customs by which
      affairs of commerce are regulated.

   General customs, those which extend over a state or
      kingdom.

   Particular customs, those which are limited to a city or
      district; as, the customs of London.

   Syn: Practice; fashion. See Habit, and Usage.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

CUSTOM OF MERCHANTS, lex mercatoria. A system of customs acknowledged and
taken notice of by all nations, and are, therefore, a part of the general
law of the land. See Law merchant, and 1 Chit. Bl. 76, note 9.