Meaning

noun slices of bread that have been toasted

noun a celebrity who receives much acclaim and attention

noun a person in desperate straits; someone doomed

originFrom Middle English tost, from the verb tosten (see below). Sense 3 is according to the Oxford English Dictionary a figurative application of sense 1 dating to 1674. It began as an epithet for a lady being supposed to flavour a bumper like a spiced toast placed in that drink. (In this context, a bumper is a drinking vessel filled to the brim.)

Etymology from Wiktionary, CC BY-SA 3.0