Common law? What the hell is that?

Geoffery Fieger:
The court of appeals' ruling just came in and they ruled in our favor. They said that the law was so poorly worded that it could not be upheld. Jack Kevorkian:
Oh. Well, I'm glad to hear there's still some smart people in power left. Geoffery Fieger:
That was the good news. They also ruled that there's no constitutional right to commit suicide... Jack Kevorkian:
I take back what I just said. Geoffery Fieger:
...and that aiding in one falls under an old common-law definition of murder. Jack Kevorkian:
Common law? What the hell is that?