What the f*&% does "should of" mean? Nonsensical. 
Depends on how you spell it. One can say "I should OFF that motherfreaker for what he said!" But even then it's misspelled.
But even this use can be a solecism waiting to happen especially if used in the conditional past perfect. "I should've offed that motherfreaker..." becomes "I should of offed that motherfreaker..."
