Common employment was an historical defence in English tort law that said workers implicitly undertook the risks of being injured by their co-workers, with whom they were in "common employment". The US labor law terminology was the "fellow servant rule".
Development
The operation of the doctrine was seen first in Priestly v Fowler in the United Kingdom. In the United States the doctrine was seen in Farwell v. Boston & Worcester R.R. Corp.
It was abolished altogether by the Law Reform (Personal Injuries) Act 1948 in the United Kingdom.
The doctrine has been superseded in the United States by worker's compensation laws, by which a worker can file for a quasi-tort, regardless of their co-worker's fault.
References
Common employment Wikipedia(Text) CC BY-SA