nigger in the woodpile n. phr.,
slang Something unexpected that changes a situation; a hidden factor or trick.
■ Racist and offensive, but commonly used in the past.
I knew there had to be a nigger in the woodpile, because the man was being much too generous. When the salesman gave him an extra tire for his bike, the boy suspected a nigger in the woodpile. [TahlilGaran] English Idioms Dictionary ▲