Wiktionary
nigger in the woodpile
n. (context offensive English) A hidden motive, influence or factor; a concealed (potential) snag; something suspicious.
Wikipedia
Nigger in the woodpile
A nigger in the woodpile or fence is a dated American figure of speech meaning "some fact of considerable importance that is not disclosed—something suspicious or wrong". Commonly used in the late 19th and early 20th centuries, usage has declined since then, and use of the phrase by public figures has often been followed by criticism over the offensiveness of the term " nigger".
Usage examples of "nigger in the woodpile".
And, mind you, unless there's a nigger in the woodpile somewhere, the woman the bishop is inquiring about on the manslaughter business is the mother of the Brownley girl.