The Collaborative International Dictionary of English v.0.48:

Privy \Priv"y\, n.; pl. Privies.
   1. (Law) A partaker; a person having an interest in any
      action or thing; one who has an interest in an estate
      created by another; a person having an interest derived
      from a contract or conveyance to which he is not himself a
      party. The term, in its proper sense, is distinguished
      from party. --Burrill. --Wharton.
      [1913 Webster]

   2. A necessary house or place for performing excretory
      functions in private; an outhouse; a backhouse.
      [1913 Webster +PJC]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PRIVIES. Persons who are partakers, or have an interest in any action or
thing, or any relation to another. Wood, Inst. b. 2, c. 3, p. 255; 2 Tho.
Co. Lit. 506 Co. Lit. 271, a.
     2. There aye several kinds of privies, namely, privies in blood, as the
heir is to the ancestor; privies in representation, as is the executor or
administrator to the deceased privies in estate, as the relation between the
donor and donee, lessor and lessee; privies in respect to contracts; and
privies on account of estate and contract together. Tho. Co. Lit. 506;
Prest. Con v. 327 to 345. Privies have also been divided into privies in
fact, and privies in law. 8 Co. 42 b. Vide Vin. Ab. Privily; 5 Coin. Dig.
347; Ham. on Part. 131; Woodf. Land. & Ten. 279, 1 Dane's Ab. c. 1, art. 6.