They've been called "quill stems" a long time. The term was hardly new when I was riding in the seventies. I suspect Merziac's got it; that those stems are anchored at their "root" by the wedge like the feather of a bird (or a hair on our head - feathers being evolved hair). Most other bike parts are secured where the emerge from the frame, like seaposts. (I doubt the term came from the similarity of the taper at the quill/feather base that you see in the photo above. The early quill stems used a conical wedge and the stem had a squared off cut with all the tapering being inside the stem. Cinelli, TTT, etc. The external wedge Japanese stems came later. The Japanese may well have been using that taper for a long time; bicycles being over a century old there but there was no crosss-over between Japan and the western market until ~1970 and the term "quill" was already well established.)
Very good answer. Thanks and I hate it when my quill gets stuck.