interloper

Etymology

1590s, from inter- + loper (“runner, rover”), as in landloper (“vagrant”) (from lope (“to leap, to jump”) (originally dialectal)). Originally spelt enterloper and used in specific sense “unauthorized trader trespassing on privileges of chartered companies”, later general sense of “self-interested intruder” from 1630s.

noun

1. One who interferes, intrudes or gets involved where not welcome, particularly a self-interested intruder.
   They disliked the interloper, and forced him to leave.
2. (obsolete) An unlicensed or illegitimate trader.