Slang phrase nigger inside the woodpile "a concealed motive or unidentified issue impacting a predicament in an adverse way" [OED 1989] is attested by 1800; Thornton's "American Glossary" (1912) defines it as "A mode of accounting for your disappearance of fuel," therefore "an unsolved mystery.In summary, using the word "Niga" is never satisfactory