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[spoiler]Double penetration, derp punch, donkey punch/pick, dragon punch, deep penetration. The list goes on.[/spoiler]
Here in Texas, it usually refers to Dr. Pepper, a delicious soda that used to be made in a special form at the Dublin Bottling Plant in Dublin, TX. It was made with real cane sugar, was delicious and could be incorporated into the baking of certain chocolate cakes for great success.
A bit too far offtopic on these, I think!
What’s wrong with it? He did ask what DP means. There are plenty of short terms things that can be contributed to DP. I was just stating some off the the top of my head.
I think it’s pretty clear what context of those two letters the OP is seeking information about.
I get that you were going for a joke, but it’s neither appropriate or on topic.
Ah yes, no no. I wasn’t sure if that anecdote was going too far or not. I’ll stick with the Dr. Pepper joke then. Thanks @xSkeletalx
I use Kim Wus dragon kicks as her DP LOL
Ok ok. No more jokes.
Shadow or regular?
The MK & FK
I think everyone has pretty much covered what a DP is by this point. If you want to know more about option selects (OS’s), see this thread:
The one I can never explain to others but equally legendary, imho: Tiger Knee. Anyone dares to explain? (I.e. anyone not named infil,lol)
A tiger knee motion (or TK) is essentially doing an air special as low to the ground as possible. This is done by inputting the motion and fluidly adding a jump into it before pushing a button.
The motion would look like 2369+button. It takes a few to get used to it, but in some cases it is 100% necessary to learn. In GG:Xrd for example, Bedman has a projectile that can be low profiled. But when he does the same projectile in the air, it has a diagonally down trajectory. So when you TK it you will essentially throw it across the ground without ever really leaving the ground giving you two separate options for your projectiles trajectory.
I always giggle when people say DP. I’m that mature. XD
There are so many terms in fighting games, and some of them mean the same thing. It can indeed get confusing.
To clarify @QSpec02 's response for anyone not familiar with keypad notation, a tiger knee’d special is 
+button
As he said, it allows you to do an aerial special very close to the ground. In KI, you’ll see this done mostly by Sadira and Hisako, for medium widow’s bite and air-ORZ respectively.
And TK comes not from Ryu but from his long time SF rival, Sagat. In case you are/were trying to look things up.
DP is basically… A Shoryuken in street fighter