Abbreviation for "you just
know."
Typically
used in
reference to subtext, often sexual, that is hidden to most but blatantly obvious to those "in the know."
Example 1
Jeff: "You seen how happy
Lindsey's been recently? It's like her mood's done a 180 since she broke up with Kenny and got that dog."
Mark, sarcastically: "I wonder why..."
Jeff: "What are you implying?"
Mark: "YJK"
Example 2
Carol: "You see that new fairy type
Pokemon in Sword and Shield yet?"
Brad: "Yeah,
Pokemon like Hatterene really put the 'Freak' in Game Freak. It's like Gardevior all over again."
Carol: "Yep, total fapbait. YJK"