Re: Surprised that you weren't commenting on the limey use of "the boot"
In the context of computers, boot is a contraction of booting, which is itself a contraction of "pulling itself up by it's bootstraps".
It's basically a way to start small programs that call larger programs that end in loading the operating system.
I suspect, but I don't know for certain, that the term boot for a car actually originates in the term caboose, which was a storage compartment at the back of a horse-drawn carriage or coach (as well as other things).
I'm prepared to be shouted down about that.
But why trunk? That is a piece of luggage, an elephant's nose, or when paired, warn by men as swimming apparel.