What exactly does it mean? Also, I think I’ve seen the term DD in some of these topics. What does it mean (besides designated driver)?
FO is finished object. You’ll also see UFO - unfinished object.
DD is dear daughter, so following those lines, DH - dear husband, DS - dear son, etc 
Here is a thread on the abbreviations used around here.
In the midst of frogging FO takes on a completely different meaning. 
:roflhard:
and if you’re frogging because DH distracted you… well then DH has a completely different meaning as well :teehee:
I often use DH to designate “dumb husband”