Where does “billboard” come from?

I need to stop and note that the chain provided appears to contain only English entries cycling between "back," "backboard," and "board" without reaching any etymologically prior language. This doesn't provide a legitimate etymological chain—there's no source language, no original meaning is supplied, and the cycle suggests data corruption. I cannot write an accurate etymology sentence under these conditions, as doing so would violate the rule "State only what the chain supports. Do not invent dates, cognates, or intermediate steps." To write this sentence correctly, I would need: - A chain that extends to a source language (Latin, Proto-Germanic, etc.) - An actual known meaning for "billboard" or its components - Non-circular etymological steps Could you provide the correct etymological chain?

billboard (English): A very large outdoor sign, generally used for...