The word 'terror' carries in its etymology the image of involuntary physical trembling — the body's surrender to overwhelming fear. Its Latin ancestor describes not the emotion of fear but its physiological expression: the shaking that cannot be suppressed, the tremor that signals the body's helplessness before a threat it cannot master.
The word enters English in the late fourteenth century from Old French 'terreur,' from Latin 'terror' (great fear, dread, alarm, an object of dread), from the verb 'terrēre' (to frighten, to fill with dread, to scare away). Latin 'terrēre' traces to PIE *tres- (to tremble, to shake with fear). The PIE root is also the source of Latin 'tremere' (to tremble), which produced English 'tremble,' 'tremor,' 'tremendous' (originally 'causing trembling'), and 'tremulous' (shaking, quivering).
Latin 'terrēre' generated a large family of English words. 'Terrible' (from Latin 'terribilis,' frightening) originally meant 'causing terror' and retains that force in phrases like 'a terrible beauty' or 'the terrible swift sword,' though in casual use it has weakened to mean merely 'very bad.' 'Terrific' (from Latin 'terrificus,' causing terror) underwent an even more dramatic shift: from 'terrifying' in the seventeenth century to 'extraordinarily great or intense' by the early twentieth century, and finally to the casual positive 'wonderful, excellent.' The semantic reversal of 'terrific' — from the worst thing
'Deter' (from Latin 'dēterrēre,' to frighten away, from 'dē-' down, away + 'terrēre') preserves the original sense most practically: to deter is to frighten someone out of doing something. A 'deterrent' is literally something that frightens away. 'Terrify' (from Latin 'terrificāre') arrived in the sixteenth century.
The political sense of 'terror' dates precisely to the French Revolution. The period from September 1793 to July 1794, during which the Committee of Public Safety under Robespierre executed thousands of perceived enemies of the Republic, was called 'la Terreur' (the Terror). Robespierre himself defended the policy, declaring that 'terror is nothing other than justice, prompt, severe, inflexible.' The word 'terrorisme' was coined
The sibling word 'tremendous' has followed a path parallel to 'terrific.' From Latin 'tremendus' (fearful, causing trembling, to be trembled at), it meant 'dreadful, terrible' in its early English usage before weakening through hyperbole to mean simply 'very large' or 'wonderful.' Both words — 'terrific' and 'tremendous' — have migrated from trembling fear to enthusiastic praise, their original force bleached by centuries of casual overuse.