A shortened version of yes ma'am, much like yessum, though it's even shorter than that. Used by them lazy folk who dang can nay type no good.
<person 1> I want you to fry the vegetables, clean the pantry, and shave your god damn balls.
<person 2> Yes'm.
This is a short form of 'Yes ma'am'. When used to a guy, it is a slight and unnoticed insult. Very good for an annoying boss.
Your mother: "Get your ass working now!"
Your boss: "Get your ass working now!"
You: ( snickering ) "Yes'm."