Easton's 1897 Bible Dictionary:

Tyrannus
prince, a Greek rhetorician, in whose "school" at Ephesus Paul
disputed daily for the space of two years with those who came to
him (Acts 19:9). Some have supposed that he was a Jew, and that
his "school" was a private synagogue.

Hitchcock's Bible Names Dictionary (late 1800's):

Tyrannus, a prince; one that reigns