In contemporary American English parlance (but not only), the words incredibly and unbelievably have all but replaced very, highly, and extremely as designations of the ultimate degree of the adjective they qualify. The fact that the literal meaning of the former––namely, ‘not susceptible of belief’––runs athwart the assertion of ultimate degree looms as a perverse semantic development: if something cannot be believed or is not credible, how can it simultaneously be valorized as obtaining for the adjective thus qualified?

The answer, of course, lies in the tendency of all languages to create new meanings through troping, i. e., by subordinating or submerging the literal in the transferred sense. Here the state of not being believable is given the figurative meaning  of ultimate degree. Ultimacy is to be interpreted, accordingly, as the state of beggared belief. The ultimate degree of assertibility, in other words, lies in a semantic space beyond believability, and it is only the tropologically established convertibility between the two states––assertibility implying believability––that conduces to the rise of the new counter-literal meaning of the two privative adverbs.