膝カックン - (hiza kakkun)
This means someone surprised me with a knee-buckling prank. “Hiza” is knee in Japanese. “Kakkun” is onomatopoeia, it’s describe the condition of when knee was buckled. In japan, junior high school students (especially girls) play it in school. I get caught more or less when stand behind partner. And it's difficult to match partner's knee positions.