"Okay" or "Alright."
It's a casual way to acknowledge something, similar to "Mm-hmm" or "Yeah." It's often used in texting, online chat, and social media posts.
The origin of the "MMM" part is uncertain, but it likely stems from a combination of:
* Sounding like a thoughtful nod or agreement.
* Similar to the "mm" used in "Mm-hmm."
* Imitating the sound of a satisfied sigh.
The "K" is simply short for "Okay."
So, when you see "MMMK" online, it's usually a signal that the person understands what you're saying and is ready to move on to the next topic.