The Collaborative International Dictionary of English v.0.48:

Attorn \At*torn"\, v. i. [OF. atorner, aturner, atourner, to
   direct, prepare, dispose, attorn (cf. OE. atornen to return,
   adorn); [`a] (L. ad) + torner to turn; cf. LL. attornare to
   commit business to another, to attorn; ad + tornare to turn,
   L. tornare to turn in a lathe, to round off. See Turn, v.
   t.]
   1. (Feudal Law) To turn, or transfer homage and service, from
      one lord to another. This is the act of feudatories,
      vassals, or tenants, upon the alienation of the estate.
      --Blackstone.
      [1913 Webster]

   2. (Modern Law) To agree to become tenant to one to whom
      reversion has been granted.
      [1913 Webster]

WordNet (r) 3.0 (2006):

attorn
    v 1: acknowledge a new land owner as one's landlord; "he was
         attorned by the tenants"