The MonkaW emote is the "zoomed in" version of the MonkaS emote (like most emotes with a "W" at the end).
But what does MonkaW mean? The MonkaW emote is used when a streamer is in an exciting situation or is facing a difficult opponent. However, it can also represent feigned worry and thus be used rather ironically. The smaller frame is meant to make the face look even more tense to express more worry.
Although it is relatively popular, it is not available on Twitch itself. To use it, the browser extensions "FankerFaceZ" or "Better Twitch TV" have to be used. But why is that?
The MonkaW emote is based on the MonkaS emote, which features the sweaty "Pepe The Frog" character from the comic "Boy's Club" by Matt Furie. Unfortunately, to the annoyance of many Twitch users, "Pepe The Frog" memes are also popular in other communities outside the gaming world. The use of "Pepe The Frog" by other groups has meant that emotes featuring the frog will probably never make it into Twitch's Global Emotes.