Why Semi's are called Semi's

So since Phil posted his ponderings about why semi trucks are called semi's since they're not semi anything... they're freaking huge... I've been playing this little game at work.

So I'm working in customer service/sales in the airport, and have to deal with a lot of frustrated (that's putting it charitably) travelers. It's very frequent that one of our customers will have an outburst, and inquire, "well WHY... blahblahblah": questions I find unpleasant to answer, because we both know they're not going to be happy with the answer. So right, I then respond in a tone reflecting how equally bewildered and cheated I feel ask, "YEAH, and WHY do they call Semi's Semi's??" (Okay, so it's not kosher exactly, but it's certainly much more fun for me.)

They then stop and look at me, surprised and confused, and up until last night, they all stopped suddenly (distracted) and realize they don't know either and we proceed to wonder and discuss. It's fun, as I said, and a useful tool for diffusing the situation.

But then last night a woman came in who's husband works with trucks (as I found out later), and of course decided to play with her. Turns out she actually had an answer! Apparently you're NOT supposed to call it a semi unless it's just the front engine part with no trailor or load attached. If it's the full deal, it's called a tractor trailor. But since "tractor trailor" is pretty cumbersome, I guess the masses (me and my previous clients) just took to calling them all semi's.

Figures.

So now I need a new question for my game. Anybody? It's gotta be hard or it won't work.