Slang phrase nigger in the woodpile "a concealed motive or unfamiliar component influencing a problem within an adverse way" [OED 1989] is attested by 1800; Thornton's "American Glossary" (1912) defines it as "A method of accounting for that disappearance of gas," for this reason "an unsolved thriller.The slang phrase "Niga" is a remarkably offensi