The symbol is a combination symbol comprised of the downwards zigzag arrow symbol ([url=https://en.wikipedia.org/wiki/Downwards_zigzag_arrow#:~:text=The%20symbol%20%E2%86%AF%20(U%2B21AF,to%20separate%20molecules)%20in%20chemistry]representing a contradiction[/url]) and the right-angle symbol (representing Pythagorean's theorem, a.k.a. the rule of 3-4-5). The resulting symbol represents Proof by Contradiction which is a well-known exercise with the Pythagorean theorem.

It was an early attempt to symbolize that meaning. Now, various symbols for contradiction are used in context to imply "proof by" contradiction and some others exist in limited venues. The symbol above never caught on and fell by the wayside except with packrats who construct tables of symbols.

You saw it here first and if it's in The Cellar it must be true.