Interloper

from WordNet (r) 3.0 (2006)

interloper
    n 1: someone who intrudes on the privacy or property of another
         without permission [syn: {intruder}, {interloper},
         {trespasser}]

from The Collaborative International Dictionary of English v.0.48

Interloper \In"ter*lo`per\ (?; 277), n. [Pref. inter- + D.
   looper a runner, fr. loopen to run; akin to E. leap. See
   {Leap}, and cf. {Elope}.]
   One who interlopes; one who unlawfully intrudes upon a
   property, a station, or an office; one who interferes
   wrongfully or officiously.
   [1913 Webster]

         The untrained man, . . . the interloper as to the
         professions.                             --I. Taylor.
   [1913 Webster]