Deep capability in one domain, built by long practice. Expertise is knowing what matters here, which is why it does not transfer.
French expertise, from expert, from Latin expertus, the past participle of experiri (to try, to test).
Its root is the same trial that gives experience, worn down by repetition until it settles into something reliable: expertus names what has already been tested and found sound. What settles this way inside one domain is exactly what refuses to travel to another.