The abbreviation "jk" stands for "just kidding." It is commonly used in informal communication, particularly in text messaging and online conversations, to indicate that something said previously was not meant to be taken seriously or that the speaker is joking.
The term "jk" emerged in the 1980s with the rise of Internet communication and texting. The abbreviation is a phonetic representation of the phrase "just kidding," and it quickly became popular in casual conversation to lighten the mood or clarify intent.