jumping-off point

Example: “That old photograph was the jumping-off point for her novel.”

Origin: A literal piece of American frontier geography before it was a figure of speech. A jumping-off place was the last settlement at the edge of organized territory — the point where roads, supply, and law ended and travelers started into country with none of them — and the term was in ordinary practical use through the nineteenth century, with towns like Independence, Missouri understood as the jumping-off point for the overland trails west. The name captures the discontinuity precisely: not a gradual thinning-out but a definite edge with a step beyond it. The modern sense, a starting point for a discussion or a project, keeps the structure while removing every trace of the risk that made the original phrase worth having.