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1.
To delibritly cause trouble or an arguement by disagreeing with someone even though you think they are right.
John looks like he could do with good shit stir today.
by james chadima January 13, 2004
 
2.
Meaning to cause trouble, disagreeing with someone, causing an argument.

Someone who likes to cause trouble, stir up arguments.
These problems would have been easily solved, but Sarah goes and shit stirs and makes a mountain out of a mole hill.
by The English Person August 11, 2008