Citronella: "How stupid, what are we meant to do? Drive slower? Donate some money? Beep our horn? What are they trying to prove. Friggen idiots."
Franz: "Citronella you are perfectly right."
Popularized in the simpsons episode where Homer joins the B Sharps, and makes these 3 words into a song.
Baby on board,
how I adore,
that sign in my car windowpane.
Folks stop and stare, friends I don't care
That little yellow sign can't be ignored
I'm telling you it's mighty nice
Each trip's a trip to paradise
With my baby on board.
It's for the Emergency Services in the event of an accident. It's so they know there may be someone in the vehicle who is small enough to have become trapped under a seat etc.
There. That's nothing to get mad about, is it?