1. informal A colloquial contraction of "yes sir," used as a more emphatic way of responding "yes." A: "Are you going to the party tonight?" B: "Yessirree. Probably going to head over there around 9 or so."
2. informal Used to emphasize what one just said, rather than being addressed to anyone in particular. Ouch, that is one nasty looking cut you've got there, yessirree! Yessirree, this new lawnmower will take care of that grass in half the time it used to take!