Answering 'OK' May Make Millennials and Gen-Zers Think You're Mad at Them (2019)
inc.com
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But if someone said "kk" I would assume an idiolect difference, not trying to prognosticate deep meaning behind it, barring other clues like chocolates. Making such inferences -- which are based on a sample of highly subjective and constantly changing idiolects which probably do not have anything approaching consensus -- is kinda setting yourself up to fail. Miscommunication can be an error of both the speaker and the listener, after all.