Possible Clues:

• It's bad to be behind it
• It's last to be sunk
• It's usually sunk last
• "Behind the ___" (in a bad position)
• Billiards variation
• The black in the game of pool
• It's immediately knocked off the table in the worst pool game
• Target on a pool table, sometimes (2 words)
• Last thing to go in a pocket, one hopes
• Rowers' function in pool?