the core of the word lies in the latin verb terrere, meaning "to frighten or terrify," which itself may be traced to a proto-indo-european root suggesting trembling or fear. this root entered english via old french terrifier to form the verb "terrify." the english derivational process then applies the "-ing" suffix to create the adjective "terrifying," describing something that induces terror. finally, the prototypical english adverbial suffix "-ly," of germanic origin, is attached, systematically converting the adjective into an adverb that describes the manner of causing extreme fear.

Structure: terrify (to frighten) + ing (adjective suffix) + ly (adverb suffix) terrify (from latin terrere, meaning "to frighten, to terrify"), ing (old english suffix forming present participles and adjectives), ly (old english suffix forming adverbs from adjectives).