I shout "Clear". Firstly, it's understandable by non-aviation people. Secondly, it applies equally in helicopters, which don't have a prop. Actually for my PPL(H) I was taught to just look behind...because it's the tail rotor you don't want anyone to run into. But I teach my students to look behind AND yell "Clear". It can't do any harm, and it might do some good.
And I don't care what anyone thinks; I'll continue to do it.
