Donnish is an adjective that means bookish.
Derived from the Latin word dominus (DOME ee noos) meaning ‘lord or master,’ a ‘don’ came to refer to leader in a number of very different contexts. It sometimes refers to a high-ranking member of the mafia, as in Don Corleone from the movie the Godfather. But it may also mean a senior professor at a college or university.
This academic meaning provides the origin of our word of the day. Donnish is an adjective that describes someone who behaves in a highly educated or bookish manner. I was surprised to learn that Jerry was a car mechanic. His donnish demeanor suggested he was some kind of scholar.