The word 恐怖 is formed by combining two characters, 恐 and 怖, which both independently carry meanings related to fear and dread. This is a common process in Chinese where semantically similar characters are paired together to create a compound that intensifies and solidifies a single core concept. In this case, each character reinforces the other, merging their individual nuances of "fear, dread, apprehension" (恐) and "terror, fright, horror" (怖) to form a stronger, more comprehensive idea of intense fear or terror. The formation is purely meaning-based, building a new, unified concept from the shared semantic field of its components.