Here's two slash commands that are not on your list. And, since they are not on the list, then there's a 99% probability there are more slash commands that are not also on this list. Anyway here they are.

/accept

/decline

I discovered these quite by accident when someone sent me a group request and the request window disappeared before I could click on it. So I typed /accept and entered the group. Then I quit the group and had the group leader send me another invite and I typed /decline and it declined he invitation.

What do DCDs stand for?

DCD = Defensive Cool Down

It is an ability that is likely to reduce the damage you take. Generally speaking. They tend to be specific to a Class, or Advanced Class. Sometimes, there will be one or more Combat Proficiencies that will make one of your DCDs better or even add an OCD (*) aspect to it. An example would be the "Force Armor" DCD of a Jedi Sage that can do a non-trivial amount of damage depending on the Combat Proficiencies chosen; helps in fights like Nefra with its DoT.

(*) OCD = Offensive Cool Down. Something that improves the tanking, damage, or healing you do; generally damage.

What do DCDs stand for?

It stands for Defensive Cool Down, as Mike says above. However, why the "cool down" part, nobody knows. They're just defensive skills that, like most skills, have a cool down before you can use them again.

It's basically just SWTOR jargon. Sort of like how many people use "cc" or "mez" to refer to a "stun" skill, even though there is no "crowd control" or "mesmerize" skill in SWTOR. (The terms come from WoW, I think.)

Or like DPS, which stands for Damage Per Second, but often just refers to a "damage dealer" character, as opposed to healer or tank.