Special test phone used by telephone repairmen. It has alligator clips on the leads so it can be quickly attached to the line connection screws on a circuit panel.
It is derived from the word "butt-in-ski" because it can be used to eavesdrop on phone calls.
For a photo, search Google Images for "butt set"
The line itself is dead. I hooked my butt set to the main panel and couldn't get a dial tone.