In a nutshell, a few common themes crop up during Internet searches on the topic of toasting. First, there's the common notion that hosts, both ancient Greeks and later Romans, publicly tested the beverage to show guests there was no poison. Apparently, having an extra ingredient in the wine was a common way to rid oneself of enemies. The clink, under this theory, came to express camaraderie. A second common urban legend simply proposes the the clink of glasses was supposed to drive away evil spirits and initiated the idea of drinking to one's health. Or was it done to simply to add the fifth sense to an evening already containing sight, smells, touch, and taste?
The truth probably has more to do with roots in by-gone rituals. Okay, let's move on to the word toast itself. Could it have been the burnt bread put into slightly off ancient wine to make it more drinkable?
Do a little searching and you can decide by yourself. If you wish, you can read a more scholarly explanation here. Leave a comment below if you find something which has to be included.