Spurious
Etymology
From Latin "spurius," "illegitimate, of illegitimate birth, false." In Roman usage it literally marked a child of uncertain fathering. English inherited both the legal sense and the broader figurative one: something whose claimed origin or validity is a sham.
adjective · /SPYOOR-ee-us/
False or fake, especially while pretending to be genuine or well-founded.
What it names
Use it for reasoning or evidence that looks legitimate but isn’t — a spurious correlation, a spurious claim of authorship — where the flaw is a counterfeit of real grounds.
Example
The study found a spurious link between ice-cream sales and drownings — both simply rise in summer.
Related idea
Harsher than ersatz: an ersatz thing is an admitted substitute for the real one, while spurious implies a deception about being real at all.