1. Literally, a respectful affirmation to a man. A: "Adams, report to your CO at 0800 hours." B: "Yes sir! A: "Will you have the report finished in time for my board meeting?" B: "Yes sir, I'm just finishing up the final points now."

2. 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, yes sir! Yes sir, this new lawnmower will take care of that grass in half the time it used to take!