I had no idea, so I googled to find something about this. This is what I found.
First, it was included as a reserved keyword.
Q: What was the entry keyword mentioned in K&R1?
A: It was reserved to allow functions with multiple, differently-named entry points, but it has been withdrawn.
(From http://archives.devshed.com/forums/c-c-134/c-programming-faqs-371017.html.)
It was never standardized; some compilers used it, in a very personal way.
It was later declared obsolete, I guess.