Well, this peculiar acronym expression stands for "Ok" or "message acknowledged". It is the same as nodding in person or saying "gotcha".
The other bit of history behind "kk" is the 1990's expressions "k, kewl". Translated, this expression meant "ok, cool", but was stylistically spelled otherwise. "k, kewl" undoubtedly also influenced the use of kk in today's online chatting.
The kk expression, like many other Internet expressions, is part of online conversation culture.
Reminder: 90% of the time, these expressions are typed in all lowercase letters. On selective occasions, you are welcome to use them in all capitals to express enthusiasm. Just remember not to type entire sentences in all caps, lest you be considered rude.
Related: 322 other common Internet expressions explained.
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