>adj<
1. the event, situation, or performance is very
good and very pleasing
2. an individual that is outstanding in some
way, or has possibly taken action that is outstanding
“Rock” is referring to “that rocks!”, which means a
good or pleasing event, situation, or performance. “Toast” refers to the result of defeating odds or accomplishing a goal, also sometimes as the intended, confident positive result of a goal. An example of this meaning is used in Ghostbusters, “This
chick is toast!”, meaning “We will very successful in defeating this
woman”, so much, in fact, her existence itself would be as
dry and crumbly as toast. In combination, then, the ‘rock’ part is used as the praise and degree of the skill involved in the “toast”, the successful result of the endeavor.