early 16c. alteration of rightwise, from Old English rihtwis, from riht (see right) + wis "wise, way, manner" (see wise (adj.)). Suffix altered by influence of courteous, etc. Meaning "genuine, excellent" is 1942 in jazz slang. Related: Righteously.
Excellent; genuine; the GREATEST: what we used to call the righteous jazz/ ''Is that righteous gold?'' ''Righteous it ain't, but gold it is'' (1900+ Jazz musicians)