terrere

The root terr- here comes from Latin terrēre, 'to frighten, to fill with dread.' A crucial warning first: this is NOT the same as terra ('earth, land'), the root in terrain, territory, and terrestrial. They look identical but are unrelated — one is about fear, the other about ground. Keep them apart.

From terrēre, Latin and then English built a whole spectrum of fear words, each at a different intensity:

- terror — the noun, the raw feeling of intense fear or dread (and, by extension, a cause of fear, even a 'reign of terror' or 'terror attack').
- terrible — '-ible' (able to) attached: able to cause dread, frightful. Over centuries it weakened from 'inspiring terror' to merely 'very bad' (a terrible meal isn't literally scary).
- terribly — the adverb, which weakened even further into a plain intensifier: 'terribly sorry' just means 'very sorry,' with no fear left at all.
- terrify — '-ify' (to make) attached: to make someone feel terror, to frighten greatly. Its participle terrifying describes the cause.

Then comes the family's most famous twist: terrific. It was born meaning 'causing terror' — terrifying. But in the 1800s it flipped completely. Through the path of 'terrifyingly big' → 'enormous' → 'enormously good,' terrific came to mean wonderful, excellent. Today 'a terrific idea' is a great one, and only the fossil phrase 'terrific speed' (frighteningly fast) hints at the original dread. This kind of reversal — a word swinging from negative to positive — is one of English's classic semantic flips.

Finally, a less obvious branch. Add the prefix de- ('away, off') and you get deter (de- + terrēre = to frighten away from doing something). A high wall deters burglars; harsh penalties deter crime. The thing that does the frightening-off is a deterrent — the word behind 'nuclear deterrent.' Here the original 'frighten' meaning is fully intact: deterrence works precisely by making people afraid of the consequences.