...someone who innocently joins a group, comes to a party, sits at a meeting--whatever--and within minutes has everyone at each other's throats
...Any Yiddish speakers here able to help out? seems a likely source for such a word!
I can think of two words that come near but don't quite hit the mark:
-- A dybbik is an evil spirit that inhabits someone's body for a time and does all sorts of mean and nasty things in his/her name, sometimes without being recognized for what it is except for the out-of-character nature of the mean acts. I believe it's Hebrew rather than Yiddish.
-- A kuchleffel (spelling will vary), literally a big cooking spoon, goes around disturbing the status quo ante (cross-thread) and often causing trouble, just because that's what cooking spoons do - stir up the pot and keep things boiling.
Maybe "kuchleffel" is the closest to the orignal query, though it doesn't quite catch the inevitablity of the strife, or perhaps the degree of disruption, either.
