jargon.artrocity.com
New Hacker's Dictionary the Jargon File v 4.4.6
[from the standard disclaimer attached to EPA mileage ratings by American car manufacturers]
1. A ritual warning often found in Unix freeware distributions. Translates roughly as “Hey, I tried to write this portably, but who knows what'll happen on your system?”
2. More generally, a qualifier attached to advice. “I find that sending flowers works well, but your mileage may vary.”