late 15c., "delirious, frenzied," present-participle adjective from rave (v.). The sense of "remarkable, fit to excite admiration" is from 1841, hence slang superlative use.
Entries linking to raving
rave v.
early 14c., raven, "to show signs of madness or delirium, to rage in speech," from Old French raver, variant of resver "to dream; wander here and there, prowl; behave madly, be crazy," a word of unknown origin (compare reverie). An identical (in form) verb meaning "to wander, stray, rove" dates from late 14c. in Scottish and northern dialect, and is probably from a Scandinavian word (such as Old Norse rafa). Sense of "talk about (something or someone) enthusiastically or immoderately" is recorded by 1704. Related: Raved; raving.