Saying

[Dumpling]

Where does the saying N***** in a woodpile come from?

[Cetti]

According to Brewer's Dictionary it was originally a way of accounting for the disappearance of fuel; it now denotes something deceitful or underhand; a concealed troublemaker or suspicious circumstance.

[WaldoMcFroog]

If you use the search facility, you will find my previous answer to this question!