It's just a silly nickname that Ross liked and the rest of the gang hated. From earlier in the scene:
Phoebe: Well, I'll tell you Rachel Karen Green, I had plans with Joey tonight and he left me this note. (Hands it to Rachel.)
Rachel: (reading the note) "Pheebs, can't make it, got a date. Talk to you later. Big Daddy." (Laughs) Big Daddy?
Phoebe: Oh that's a nickname we were trying out.
Ross: Hey, y'know what nickname never caught on? The Ross-A-Tron! (Monica shakes her head in disgust.)
Seems Ross tried out this nickname before and it didn't go so well. Joey is upset that they might have to call Ross by that stupid name again, but Chandler silently assures him that they won't.