In chess, "pig" is a slang term for a rook that reaches the seventh rank. Two rooks on the seventh rank are often referred to as "pigs on the seventh" or "blind pigs on the seventh" because of their ability to "eat" up all the opponent's pawns that are still on that rank (their starting rank). But even one rook reaching the seventh rank can be quite dangerous, especially in combination with another piece such as a queen or a bishop, so when a rook reaches the seventh rank, it is often jocularly said to have "promoted to a pig."