Source : Webster's Revised Unabridged Dictionary (1913)

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 interlopes; one who unlawfully
   intrudes upon a property, a station, or an office; one who
   interferes wrongfully or officiously.

         The untrained man, . . . the interloper as to the
         professions.                             --I. Taylor.

Source : WordNet®

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