What's "Shemp" here stand for?
"Shemp" is referencing an American comedian and actor named Shemp Howard. Shemp Howard was known for being one of the members of the comedy team the Three Stooges. If you haven't seen the Three Stooges, it's a black and white comedy show that aired from 1920 to about 1970. It's a show about three men who are always getting themselves into trouble in funny and unlikely situations. Rachel is informing the person on the phone that she has one of the Three Stooges in her office, meaning that Ross is causing problems and getting himself into trouble.