means Bread browned by heat, or the act of raising a glass to honor someone, or—informally—being utterly doomed.

from From the Latin 'tostare,' to scorch or parch, via Old French 'toster.' The drinking sense came later and literally: people once floated a piece of spiced toast in their wine, and the custom of honoring a lady by drinking to her led wits to call her the 'toast' that flavored the cup. The slang 'you're toast' is a much more recent American flourish—charred, finished, done for.