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1.
1) A rhythmic clapping sustained by a large group that begins slow and gradually builds speed before breaking out into full applause. Also known as the "unity clap", the solidarity clap is a tool used mostly by activists and labor unions to build and show unity.

2) The common affliction of a large group of activists who all sleep with one another, and eventually end up sharing one or more STDs.
1) "The solidarity clap is like our organizing -- it begins disorganized and slow, but as it picks up speed we become more and more united."

2) "I'm getting kind of itchy down there. Did you hook up with X?"
"Yeah, and zhe hooked up with Y and Z."
"Damn, looks like we've all got the solidarity clap."
by flightsongs October 02, 2011