The Brief: K stands for "okay."

Bluesquiggle
DEEP DIVE

K stands for “okay.” In texting, it is generally considered a curt or passive aggressive response due to its failure to convey enthusiasm. K can also be used to communicate irritation, annoyance, or ironic indifference; such uses include “k thanks” or “k bye.” If K were an emoticon, it would be the bored face 😐