Meaning
What does D: mean?
Origin
What's the origin of D:?
D: had been one of the earliest character combinations used as an emoticon already back in the age of SMS.
It had spread to several chatting platforms, so it is quite natural, that it occurs on the chat section of the streaming platform as well.
Spread & Usage
How did D: spread?
The popularity of the emote is undebated; even a fresh user on the internet knows how to use this emote, that is so easy to type.
It has been the matter of discussion over on Reddit as well and is seen on all kinds of websites to give expression to feelings of shock and surprise.
It is so widely known and liked, that the blue face can even be seen on various kinds of merchandize, such as mugs, T-shirts as well as laptop sleeves.

External resources
- Urban Dictionary โ D: