In the context of this thread, I meant to indicate English-speakers in the U.S., in contrast to Hispanics/latinos.
I am very well aware that obscenities vary widely in the Spanish-speaking world. There are words that are fairly innocuous here in Panama (coger, concha) that will get you in big trouble in Argentina. Likewise, a word that means “a kind of tamale” in Guatemala means “armpit odor” in Colombia and “female genetalia” here in Panama.
At least in my region of Tamaulipas (but I can say for sure in most of Mexico) and in the Lower Rio Grande Valley, that’s the tone sequence means “Chinga Tu Madre, Cabron!.”
As to why it came to be associated with that phrase, I have no idea. Still, it comes handy when someone cuts you off in the highway and the only thing left you can do is honk “dum-de-de-dum-dum . . . dum-dum.” Believe me, the other person will understand. 
XicanoreX
Haw!
Mr emilyforce ALWAYS honks the horn in a “shave ‘n’ a haircut” upon leaving his grandma’s house, even if I’m the one driving. I asked him about it once – apparently it’s a family tradition, meant innocuously, and if he didn’t do it she might even be offended. If she only knew!
Wow. I came into this thread to relate that it does indeed mean something really bad to many Hispanics, and find myself quoted as a source! I feel so validated.
It’s also interesting to find that my friend was telling the truth, and find out what it’s supposed to mean. I was guessing pretty close to the truth.