I believe in German (possibly also other languages) the thunderbolt ↯ is commonly used to mean "this is a contradiction" in a mathematical proof, equivalently to in English a kind of ⋕ rotated by 45° or the symbol ※. The symbol ⟂ on the other hand means "false" and is used in particular in formal logic.

Yes and no.

Yes, we indeed used and afaict still use the thunderbolt for contradiction in my German university.

However "perpendicular" and "bottom/falsum" are two different Unicode codepoints with very similar glyphs.

https://en.m.wikipedia.org/wiki/Up_tack

I've seen it used for contradiction. Though that's not the same thing as 'not' and I can't think of why you'd combine this with orthogonality.

If the thunderbolt means not, and the right angle is displaying the x and y axis, then this symbol could be a pun for "not a function".

High school physics and math as a major. I could scan you my scripts and papers if you're interested.....no won't. ;-D

But maybe "commonly used" was maybe the wrong term. More appropriately: "sometimes" or "by some".

Where in the world? I’ve never used it despite similar background. Perhaps regional?

I have never seen it used as not once in maths or physics. "extremely rarely" perhaps.

To be fair, there are lot of math symbols out there.

http://mirrors.dotsrc.org/ctan/info/symbols/comprehensive/sy...

There are lots of examples of the lightning bolt in there. In fact, under ulsy Contradiction symbols, there are four variants.

I also noticed the exact symbol being discussed is listed under "Angles".