If you are male, it refers to one of your testicles, usually the itchest or sorest. If you are female then it refers to one of your breasts, usually the smaller or most sensitive one. Using it in plural refers to both organs / glands.
"Don't even try to kick me in my undercutters, beyatch!" or "My right undercutter is really sensitive today! Don't fondle it so hard!"
Someone who tries to steal your business by offering the same service/product for less money.
Javi Rox: Yo, I got these cheeseburgers! I sell them to you for a dollar!
Jay Nyce: I got those same cheesburgers for 50 cent!
Javi Rox: I hate that motherfuckin undercutter.