Comes from the combination of the words "bonus" and "toast." Legend says the word was first uttered at the discovery of an unexpected extra piece of toast. Today, it is commonly used as 1) an exclamation of excitement; 2) as an adjective toward the cause of the unexpected emotion; or 3) as a direct noun, Bonus Toast.
1. "Wait, Obama is president? You mean we finally got a black man as president? Boast!"
2. "That shit was boast when the guest performer came onstage."