der Sachverstand — understanding of the matter. The word for expertise is built on Sache, the matter itself.
vom Fach sein — to be of the subject, to work in the field. Er ist vom Fach carries weight in an argument.
der Fachmann, die Fachfrau — the specialist.
Fragen Sie den Fachmann. — Ask the specialist.
Da müssen Sie den Experten fragen. — You will have to ask the expert about that.
Das kann ich nicht beurteilen. — I cannot judge that.
der Fachidiot — the specialist idiot, someone with deep knowledge of a narrow field and no grasp of anything beyond it.