A fountainhead is the origin of something — the spring at the top of the stream, used as a picture for the first source of an idea, a movement, a tradition. People say Greece is often called the fountainhead of Western philosophy. It is formal and a little literary. In ordinary talk, source, origin, or starting point will do the same work without sounding like a textbook. The picture is a fountain’s head: the place the water first comes out, before it becomes a river. Head here is the “start of a flow” head, as in the head of a river or headwaters — not, in this compound, the head on your shoulders, though that is of course the same old word stretched. Together, fountain + head = the source from which the rest flows.