The title is short for "Motherf--ker." Bono was interested in how the word came to be widely used to describe so many different things.
"It's an extraordinary thing playing 'Mofo' live," Bono describes in the book U2 by U2. "The song would come to a shuddering halt and there I was, just speaking to my mother in front of fifty thousand of my closest friends. Some nights it would really surprise me what an emotional place I would get to. Mother, am I still your son? You know, I've waited so long to hear you say so. Mother, you left and made me someone. Now I am still a child but no one tells me no. It's a device you can imagine in theatre or cinema but you rarely see in songwriting, where you stop time and the character turns to the camera and explains himself. It's Hamlet's soliloquy but I never saw any ghosts. I'm looking forward to meeting my mother. I'm fully expecting to, in the proper place and time. I hope I have a backstage pass."
The lyric contains some references to Bono's mother, who died when he was 14.
This was a new sound for U2, as they were experimenting with Techno. Disc jockey Howie B assisted with the production.
"Piano Man" was inspired by Billy Joel's time playing at a piano bar in Los Angeles. The "real estate novelist" was a guy who always talked about writing a book, but spent all his spare time in the bar.