- by virtue of office or official position.
1532, "in discharge of one's duties," lit. "out of duty," from L. ex "out of" + officio, abl. of officium "duty" (see office). (http://dictionary.reference.com/browse/ex+officio)
"To outline military relations and revolutionary goals, he also formally reorganized the rebel high command as the Revolutionary Junta of the Center and South of the Republic. He himself, as chief of the rebel army, was ex officio president of the junta."
- John Womack, Zapata and the Mexican Revolution