(n.) One who by gradual steps enters on, and takes possession of, what is not his own.

Dutch uncle Paul Polonius admonisher adviser backseat driver butt-in confidant consultant counsel counselor crasher encroacher expert gate-crasher guide infiltrator instructor interloper intermeddler intruder invader kibitzer meddler mentor monitor nestor nose orienter pragmatist preceptist prier quidnunc rubberneck snoop teacher trespasser unwelcome guest