Dating someone of a much lower caliber than you have in the past or are worthy of.
Friend #1 - 'Wow! She left a Biochemist with a Ph.D and a six figure salary to date a drugged out, abusive, divorced waiter who can't keep a job!'
Friend #2 - 'Yeah, no kidding... She was REALLY dating down with him! Good thing she finally got it together and left that loser!'