His nickname "Bono Vox" – usually shortened to "Bono" – is a corruption of Bona Vox, a brand of hearing aid for which the Latin translates to "good voice". "Bono Vox", however, literally means "The voice to the good man", Vox, the subject and Bono, the indirect object.
The word bono is also Italian slang for "sexy" and the dative form of the Latin word bonus; see List of Latin phrases.
Hmmm... sexy, eh?? ;) But mainly "The voice to the good man" seems to suit him VERY well, I believe!