(n.) One who intrudes; one who thrusts himself in, or enters without right, or without leave or welcome; a trespasser.

arrival arriviste black sheep blemish burglar busybody buttinsky comer crasher emigrant encroacher entrant foreign body foreign intruder gate-crasher gatecrasher greenhorn immigrant impurity in-migrant incomer infiltrator interloper invader kibitzer meddler misfit monkey wrench mote new arrival new boy newcomer novus homo oddball parvenu recruit rookie settler sliver snooper speck splinter squatter stone stowaway tenderfoot thief trespasser uninvited guest unwelcome guest upstart visitant visitor weed