I believe it is dear. Some have said darling. On a WDW board I am on they say it stands for disney. I guess if you are angry at him/her it could mean dang. As long as we all know that you are referring to your spouse, I guess it doesn't really matter.
dd means dear daughter. mil - mother in law and bil, sil, and fil, are brother, sister, and father in law. df would be dear father. dk means dear kids. ds dear son.
Oh, and I like the SWMBO thing.
Oh, and don't feel bad, we all just kind of figure it out (and make it up) as we go along. You are not the first person to ask on a board and for each person that has the guts to ask, there are about 100 out there saying "Thank God he asked, now I might know what these people are saying!"