A bit late to the party, but I use "Firetruck" particularly in personal emails originating from my work email address, due to the prudishness of the incoming mail server - if people reply and I've written the real word in there, it will bounce. I've used firetruck for ages - particularly around very young, innocent ears who don't need to hear the other.

I suspect that "trucking" could have come from Firetruck, and a nice bit of rhyming to boot!