Slang phrase nigger while in the woodpile "a hid motive or unfamiliar element impacting a circumstance within an adverse way" [OED 1989] is attested by 1800; Thornton's "American Glossary" (1912) defines it as "A manner of accounting with the disappearance of gasoline," hence "an unsolved secret.In conclusion, the use of the phrase "Niga" isn't app