What does the expression «to wash someone’s bones» mean?

This expression appeared in the mid-19th century. Russian writers readily used it to describe someone gossiping behind a character's back. For example, the main protagonist of Chekhov's story ‘From the Notes of a Hot-tempered Man’ becomes an unwitting witness to an unpleasant scene: "After a long conversation about love, one of the girls gets up and leaves. The remaining girls begin to wash the bones about the one who left. Everyone finds her stupid, intolerable, ugly and that her shoulder blade is out of place."

The tradition of literally washing bones does, indeed, have pagan origins. In ancient times, people believed that a deceased sinner could return to humanity as a ghoul or vampire. To prevent this, their remains were dug up and the bones washed with water. This tradition was partially preserved among some peoples, including Slavs. For example, in the 19th century, church historian Yevgeny Golubinsky described a similar tradition in the Greek Orthodox Church: bones were removed from the grave, washed and, after a funeral prayer, transferred to a special burial vault – a ‘kimitherion’ (lit. ‘sleeping chamber’).

Over time, the expression "to wash the bones" came to be used to describe a conversation about someone's qualities and actions – not always in a positive light. More simply, it referred to the exchange of gossip about someone behind their backs.

An English equivalent would be: “To pull someone to pieces (behind their backs).”