Do you remember?

In the Korean comments

One comment turns Yuha’s appearance into a full princess-and-knight fantasy. It reads, in translation, “Seeing Yuha makes me feel like I’ve become a knight who would endure any hardship to serve and protect a real princess.”

What the words mean

심장 멎음 (simjang meogeum, literally “heart stopped”) is casual, exaggerated shorthand for being momentarily stunned or overwhelmed. Here, one commenter uses it for the moment Yuha entered through the door, not as a literal medical statement.