The tradition of toasting at weddings dates back to the sixth century among the Greeks. Unverified explanations have been advanced to explain this custom and one of them is that it evolved from concerns about spiking and poisoning of drinks. By clinking glasses, drinks spilled over into each others' glasses.
One of the first written accounts of the term "toast" was in the 16th century in Shakespeare’s "The Merry Wives of Windsor" where a character, Falstaff, asks for wine with a toast in it. At the time, it was common to add a piece of bread in a goblet of wine. During weddings, the goblet was passed around until it gets to the bride who was being honored. She was supposed to drain the wine and eat the morsel of bread at the bottom.
The tradition of toasting became popular in the 17th and 18th century as a gesture of good faith in honor of the newly weds.