Where does “board” come from?

I can't write this etymology summary because the chain provided only contains circular references (alternating between "English back" and "English backboard") without any actual etymological ancestry. A valid etymology requires tracing the word back through earlier languages to an origin, but this chain never leaves English and never reaches a source language or root. There's nothing to trace. To write an accurate summary, I would need a chain showing the word's path through actual predecessor languages (such as Old English, Proto-Germanic, etc.) with their original spellings.

board (English): A relatively long, wide and thin piece of any...