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1.
A yellow sign that shit-for-brains drivers believe will cause others to be extra cautious around them. In reality it signifies an idiot that thinks they're special because they have the sign that partially obstructs their vision, whether they have a baby in the car or not. It does NOT mean they are being careful while driving or give a shit about you. It actually can be viewed as a point of reference indicating you're within the vicinity of a fucking idiot behind the wheel.
I sometimes take the opportunity to pull along side of someone displaying a Baby On Board sign to see for myself what a fucking moron looks like. These cretins justify the issue of not allowing some imbeciles permission to breed. Do they actually think anybody gives a shit about their fucking kid?
by Big Ed Moustapha March 02, 2010
 
2.
a stupid freaking yellow sign that most commonly mothers put on their car window to signify the fact that a baby is "in the car" Which basically means drive carefully which you should be doing anyway and who cares? Good on you for having a baby, you dont have to share it with the world because surprisingly no one else cares.
Franz: "Oh look, those people have a baby on board, i can tell by that yellow sign that hangs in their window."

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."
by emmo (not emo) August 15, 2006
 
3.
The quick translation: Bitch on Board
"Baby on board" indicates a woman driver with a huge sense of entitlement just because she managed to pop out a kid. Stay out of her way. She thinks she had the immaculate conception, and expects everyone to bow down to her kid as if it were the new Baby Jesus.
by drunk oracle March 19, 2010
 
4.
A sign that can often be seen in car windows indicating that a baby is riding in the car.

Popularized in the simpsons episode where Homer joins the B Sharps, and makes these 3 words into a song.
The 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.
by Anonymous September 21, 2003
 
5.
Contrary to other definitions posted here, displaying a 'baby on board' sign is not actually intended to be an attempt to make other drivers drive more carefully.

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?
See that mangled wreck? How cool is that? OMG, there's a Baby on Board!
by Mathilda Underfoot March 08, 2010