toast (v.1)

"to brown with heat," late 14c., tosten, from Old French toster "to toast, to grill, roast, burn" (12c.), from Vulgar Latin *tostare (source of Italian tostare, Spanish tostar), frequentative of Latin torrere (past participle tostus) "to parch" (from PIE root *ters- "to dry"). Related: Toasted; toasting.

"piece of bread browned by fire or dry heat," early 15c., tost, from toast (v.1); originally as something added to wine, ale, etc. It is attested from 17c. as something eaten on its own with a spread. Tostie is attested from late 14c. as "toasted piece of bread, dish made with toast."

The slang meaning "a goner, person or thing already doomed or destroyed" is recorded by 1987, perhaps from the notion of computer circuits being "fried," and with unconscious echoes of earlier figurative phrase be had on toast (1886) "be swindled," on the notion of "be served up for eating." To have (someone) on toast was to have the person at one's mercy (1889). Earlier was served up on toast (1842). Other sources trace the extended sense and popularity to its use in the 1984 film "Ghostbusters."

"a call to drink to someone's health," 1690s (but said by Steele, 1709, to date to the reign of Charles II), originally referring to the beautiful or popular woman whose health is proposed and drunk to. The custom apparently grew from the use of spiced toast (n.1) to flavor drink; the lady being regarded as figuratively adding piquancy to the wine which was drunk to her health. 

The custom itself is much older than this word for it, and the expectation of a bit of toast in a mug of ale at a tavern is well attested in many 17c. drinking songs, though none of them seems to give a reason for it. 

Steele's story ["Tatler," No. 24] is that an (unnamed) beauty of the day was taking the cold waters at Bath, when a gentleman dipped his cup in the water and drank it to her health; another in his company wittily (or drunkenly) replied that, while he did not care for the drink, he would gladly enjoy the toast. The meaning "one whose health is proposed and drunk to" is from 1746.

Toast-master, appointed to propose or announce toasts at a public dinner, is attested from 1749.

"to propose or drink a toast," 1700, from toast (n.2). This probably is the source of the Jamaican and African-American vernacular word meaning "extemporaneous narrative poem or rap" (1962). Related: Toasted; toasting.