interloper

[in-ter-loh-per]
noun
1.
a person who interferes or meddles in the affairs of others: He was an athiest who felt like an interloper in this religious gathering.
2.
a person who intrudes into a region, field, or trade without a proper license.

Origin:
1585–95;inter- + -loper (see landloper)

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
interloper (ˈɪntəˌləʊpə)
 
n
1.  an intruder
2.  a person who introduces himself into professional or social circles where he does not belong
3.  a person who interferes in matters that are not his concern
4.  a person who trades unlawfully
 
[C17: from inter- + loper, from Middle Dutch loopen to leap]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

interloper
c.1590, "unauthorized trader trespassing on privileges of chartered companies," probably from inter- "between" + -loper, from landloper "vagabond, adventurer" (also "a term of reproach used by seamen of those who pass their lives on shore" [Johnson]), from M.Du. loper "runner, rover," from lopen "to
run." General sense of "self-interested intruder" is from 1632.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
That's a security guard moving in at right to intercept the interloper.
The nitrogen interloper provides an extra electron which can be used to
  generate photons or to store quantum information.
If an interloper manages to evade the cameras and somehow get inside, he will
  face other high-tech security measures.
Loosestrife has gone from being a nuisance and an interloper to a botanical
  bully and a green cancer.
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