It's interesting, by the way, that I had to look up the word kumzitz in the Urban Dictionary and not an online Yiddish dictionary. Fortunately, Bashalon, the Hebrew Language Detective, can explain.
"It is very common around the Lag B'Omer bonfire to have a sing-along called a kumzits (also spelled kumzitz or kumsitz). As this article describes, kumzits is an unusual word. On the one hand, it is widely known that the origin is from Yiddish for "come [and] sit". However, the word does not appear in Yiddish dictionaries. Why is that?Hat tip to YouTube user HolyBrotha for posting a video of a kumzitz (and lots more) and to Balashon for explaining the history of the word. Balashon is a good read, check it out. Finally, just to be honest. I can' read the article that Balashon references. My Hebrew isn't nearly that good. But he linked to it and so I did to. Ignorant in public, that's me!
It turns out that kumzits is a Yiddish word that exists only in Hebrew. It was adopted by the early pioneers in Israel, despite the establishments opposition to use of Yiddish words. Hebrew replacements were suggested such as shevna שבנא - "please sit" and the Talmudic tozig טוזיג. But nothing ever managed to displace kumzits from its place beside the fire."