whatever (pron.)
mid-14c., "what in the world," emphatic of what, with ever. From late 14c. as "anything at all; all of; no matter what or who." From late 14c. as an adjective, "any sort of, any, every; no matter what, regardless of what." From 1870 as "whatever may be the cause, at any event," which could be the source of the modern teen slang dismissive use.