A dipping tobacco spitter that is used and then put away for some time only to be found without the knowledge of whose spitter it is. Oftentimes these spitters are reused for no reason other than not having to chug an entire bottle of water for a clean spitter (and a guaranteed bathroom break). Mystery spitters are most commonly found in Preparatory School settings.
"What spitter can I use for this lip?"
"I don't know man, I guess you'll have to grab a mystery spitter.