Meaning
What does MonkaS mean?
Part of the BTTV (Better Twitch.tv), which is an extension to your browser to enhance your viewing experience, on the streaming website Twitch.tv, by having access to new features like emoticons and chat functions, etc.
MonkaS is a variant of the meme FeelsGoodMan, which both feature Pepe the Frog, in this case, he is portrayed as sweating and nervous.
MonkaS can be used to express tension, for example when the streamer is playing a horror game on screen and a scary part is up.
MonkaS is part of the Pepe the Frog variant emotes which are as following:
Origin
What's the origin of MonkaS?
The earliest known mention/use of the picture was in 2011, which was then later on turned into a unique emote for the streaming website Twitch.tv and added to the browser extension called FrankerFaceZ on 2016.
Spread & Usage
How did MonkaS spread?
As of 10/12/2019 according to the website StreamElements, MonkaS is the 16th on the list of most popular/used emotes on the streaming website Twitch.tv, making it one of the most popular variants of the original emote FeelsGoodMan, which both feature Pepe the Frog.

External resources
- Know Your Meme โ MonkaS