The act of forcefully ejecting saliva from your mouth, or in slang, rapping or speaking with passion and skill.
From Old English 'spittan,' possibly imitative in origin because it sounds like the actual action. Germanic languages have similar sounding words.
In hip-hop culture, 'spitting' bars means rapping so skillfully that it's like spitting fire—the metaphor took over so completely that young people use 'spitting' to mean excellent rapping.
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