Why do we say "to boot"?

Here's an example of the phrase "to boot":

My wife made a disgusting looking dinner, and it tasted awful to boot!

The implication of the "to boot" is that the fact that the dinner tasted awful was as disappointing (or perhaps more so) than the fact that it looked disgusting.

It's a colloquial, possibly slang phrase, but I'd like to know what it's origin is, and whether it perhaps has other more esoteric uses.

Nowadays, "to boot" is simply an idiomatic way of saying "moreover, on top of that" (see e.g. Wiktionary). Originally, it comes from Old English to bote. As Etymonline explains, in Old English bot meant "'help, relief, advantage; atonement,' literally 'a making better,'" from Proto-Germanic *boto, which is also where the word better comes from.

It seems that to boot simply meens in addition.

My wife made a disgusting looking dinner, and it tasted awful too!

The origin would be bat meaning useful, so the original meaning would be used only in a positive sense.

phrases.org: to boot
thefreedictionary.com: to boot