An experiment which is expected to fail disastrously.
The original trucksperiment involved jumping in the back of a truck and rolling it down a steep hill, but the word has since come to reference any such experiment that is expected to go badly.
"Lets see if my snake gets along with your pet rabbit, it'll be a trucksperiment."
"I'm doing a trucksperiment to see how drunk I can get off of this rubbing alcohol."
1. An experiment that is expected to fail.
Johnny's trucksperiment with using ice cream as an antifreeze didn't go so well.