It stands for "[Prontor-Compur](https://en.wikipedia.org/wiki/Prontor-Compur#Etymology) connection":

> "Prontor" has its origins in the Italian word "pronto", meaning ready (and was a leaf shutter made by Alfred Gauthier). "Compur" is derived from the word "compound" (the "Compound" was a long-lived series of leaf shutters made by Friedrich Deckel).