"Scared silly" is an idiom that refers to a state of extreme fear or terror. There are many synonyms for this phrase, depending on the context and level of fear. One synonym could be "terrified," which suggests a feeling of extreme apprehension and dread. Another option could be "petrified," which means being so scared that one is frozen with fear. Other synonyms could include "panicked," "frightened," "horrified," or "spooked." These all convey a sense of fear and anxiety that can be overwhelming. Ultimately, the choice of synonym will depend on the writer's tone and intention, as well as the circumstances of the situation.