A spitting blunt is when a marijuana cigar (blunt) is "spitting" out marijuana when you inhale on it. It is common curtosy to let the person you are passing the blunt to know it is "spitting". This can sometimes be difficult if your own your last hit and it "spits" on you. In this case, you coughingly say "its spittin".
Some marijuana comes out of the blunt and Jimmy says "Its spittin"