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1.
When at least semi-experienced musicians get together and pull songs out of their ass, working all together non-verbally. Almost always involves hilarious bullshit lyrics.
Good jam sesh last night man, way to pull that riff out of your ass
by bassjesus March 19, 2014
 
2.
Subconsciously dancing to a song then noticing that others are doing the same, which only makes the group jam more enthusiastically.
The Black Eyed Peas can usually entise a jam sesh.
by ahhhsuraah January 19, 2011