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]
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]