If you intrude on people without their permission, you are an interloper. An interloper crashes parties and laughs at "No Trespassing" signs.
They figure the word interloper was formed by combining inter- ("between") with -loper (from landloper — meaning "vagabond or adventurer"). So, it only makes sense that an interloper is someone who has ventured or traveled where he was not welcome. The noun interloper may also refer to something other than a person: "The new chain superstore built at the edge of town and was an interloper among the various mom and pop grocery stores in the area."
