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.

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".