1939, "clumsy, careless, sloppy," U.S. Army Air Corps slang, of unknown origin. Origins among cadets in Texas suggest possible connection to Mex.Sp.
rancho (see
ranch), which had connotations of animal filth by 1864. Sense of "coarse, vulgar, smutty" is from 1967.
Raunch (n.) is a 1964 back-formation.