How does one get raunchy?

Etymonline suggests that its origin may come from the Mexican "rancho", a term which carried a connotation of filth:

Raunchy (adj.):

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

An instance of earlier usage is from "So you're writing a play!" by Clayton Meeker Hamilton Little, Brown, and company, 1935

Raunchy 1935:

• These plays were written in a raunchy, post-naturalistic style, or presented as political revues and satires. They were written almost entirely by young men..

• also according to Ngram the term was used from the 30's, earlier instances do not appear very meaningful.

Its original meaning from the 30's is:

• Inept, incompetent, sloppy; unpleasant, contemptible, mean, disreputable; dirty, grubby. -

  • 1939 Forum & Century July 45/1 Depending on how good or how ‘raunchy’ we [sc. Air Force cadets] were, we drilled from one to three hours in the torrid heat.
  • 1939 Air Corps Newsletter (War Dep.) - Raunchy - a name applied to anything that is dirty or in bad shape.

  • 1949 Cavalier Daily (Univ. of Va.) 22 Oct. 4/1 This situation could become embarrassing—if the writer in question happened to be well-known as a somewhat raunchy character in reality.

  • 1953 Berrey & Van den Bark Amer. Thes. Slang (1954) §759/2 Raunchy flying, clumsy flying technique.

Its later meaning

• (of persons,) their actions, etc.: boisterous, earthy, sexually provocative, aggressively licentious, suggestive. Also in extended uses, esp. of language, humour, songs, etc.: bawdy, salacious, smutty; tending to excite sexual feeling, appears to have developed during the 60's:

  • 1967 ‘E. Queen’ Face to Face iv. 17, I fell in love with him. In a raunchy sort of way he's beautiful. 1969 Sat. Rev. (U.S.) 31 May 44/2 A blend of raunchy humor, unpleasant perversity, and..sickening brutality.

  • 1970 Melody Maker 12 Sept. 7 Most of the songs were too twee, and the rest seemed to be too raunchy. 1971 Sunday Australian 7 Nov. 27/6 Russell now has two albums of his own on which to disport his raunchy rock compositions.

  • 1973 Daily Colonist (Victoria, B.C.) 23 Nov. 23/1 A drunk at the next table was singing some raunchy songs. The songs kept getting dirtier and dirtier.

  • 1974 T. P. Whitney tr. Solzhenitsyn's Gulag Archipelago I. i. i. 21 They had noticed two raunchy broads going to bathe. -

Another hypothesis is that the term come from an older US slang very similar term:

Ranchy: a.U.S.slang.n(OED)

• Dirty, disgusting, indecent.

• 1903 A. M. Binstead Pitcher in Paradise xii. 283 Then they brought the monkey in—the sad-faced, bare-based, flea-ranchy old monk.