It comes from the italian word "bimbominkia" (bimbo => baby; minchione => dumb).
Dumbaby means:
- that sort of teenager who go jumping around, ripping off his/her hair, screaming out loud at the concert of some freak guys who've just got famous thanks to some brand new freak song.
- he thinks he's so cool and skilled surfin' the internet with the father's brand new pc and trying to hack the world with some msn-account-freezing program or some oher stupid cheat.