It's "hmm" and "kkkk" put together. I searched for some tweets using it and I came to the conclusion that it's just a normal way of laughing on the internet. I had never seen or used it until now, so please take this answer with a grain of salt. EDIT: Thinking about it, it can also be used like this: Friend A: Do you remember that guy I met at Rebecca's party? Friend B: Which guy? Friend A: The one wearing a blue shirt. Friend B: hmkk Yeah, I do. Friend A: He DM'd me today asking if I'd like to go out with him tomorrow... This usage of "hmkk" KIND OF has a feeling of flirtation/excitement.

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