People who hop onto the ride (your plans, trip, or outing) in a very last-minute way. Most likely delaying, fucking things up, being a pain in the arse, or dragging everyone else down.
Jeff: Ok, so party plans are set and there isn't going to be any extra baggage coming along right?
Jeff: You sure?
Jeff: really sure?
Bob: Look, if you're so worried, let's just keep our lips sealed about the party, and reject any later requests, you know, extra baggage.
Jeff: (thinking of Sharon)...unless they aren't really considered extra baggage right?
Bob: Then you take them at your own responsibility, ok?
Jeff: Yeah, sure, ok.