“Sussy” grew out of “sus,” the shorthand for suspicious that spread widely through Among Us in 2020. As the game exploded on streaming platforms, people started using “sus” and “sussy” far beyond the game itself. This sound fits that wave of short reaction clips built around accusing someone of acting off. It remains popular because the phrase is instantly understood online, especially in gaming humor, and because it can be dropped into almost any clip where someone looks guilty or obviously unconvincing.