Green’s Dictionary of Slang

1. a white person.

1912		F.M. Hueffer Panel I. i. 14: ‘Do you mean to say that you haven’t got a single book of James’?’ ‘Never heard the name,’ the bookstall boy said. ‘But there’s plenty by Mrs. Kerr Howe.’ ‘That kelch!’ Major Foster exclaimed [OED].
1929		C. McKay Banjo 103: ‘What a saucy-looking doll that one is!’ Banjo exclaimed. ‘I ain’t studying any kelts,’ replied Taloufa. [Ibid.] 107: I ain’t got any appreciation at all for the kelts.
1939		D. Burley N.Y. Amsterdam News 20 May 20: [O]lenty of ritzy kelts from downtown coming up to see her!
1949		Monteleone Criminal Sl. (rev. edn).
1971		(con. 1930s) B. Short Black and White Baby 37: Another long-gone expression that my brother Bill used to use was: ‘three-quarters Kelt with molly-gloss hair,’ which meant a colored person with fair skin and straight hair. [...] ‘Kelt’ was Negro slang for a white person, and the ‘Molly’ in molly-gloss has some sort of Scotch-Irish connotation.

2. used to mean a light-skinned black person, in phr. such as three-quarter kelt.

1933		Eve. Sun (Baltimore, MD) 9 Dec. 31/5: Three-quarter kelt — a mulatto.
1934		J.L. Kuethe ‘Prison Parlance’ in AS IX:1 27: three-quarter kelt. A light-skinned Negro.
1938		C. Himes ‘Pork Chop Paradise’ in Coll. Stories (1990) 265: Then he met a high-yellah gal, a three-quarter keltz, from down Harlem way, and she sent him to the dogs.