K-Word
The K-Word is that ex who turned your life into a nightmare. She acted like she was the best thing ever, but you were just her emotional doormat. Now you call her the K-Word because you're too scared to say her name out loud.
"I dated the K-Word and it was like being trapped in a horror movie."
"He still calls her the K-Word like she'll come back to haunt him."
"Don't even bring up the K-Word. It'll make him cry."