terrorism (n.)

c. 1400, "causing terror; that excites or is fitted to excite awe or dread; frightful; unendurable," from Old French terrible (12c.) and directly from Latin terribilis "frightful," from terrere "fill with fear."

This is reconstructed to be from PIE root *tros- "to make afraid" (source also of Sanskrit trasanti "to tremble, be afraid," Avestan tarshta "scared, afraid," Greek treëin "to tremble, be afraid," Lithuanian trišėti "to tremble, shiver," Old Church Slavonic treso "I shake," Middle Irish tarrach "timid").

Also used in reference to the sources of feelings akin to dread, hence, by 1590s, "violently severe" etc., weakening by 18c. to a mere intensive, "great, severe" (a terrible bore; compare similar evolution in awful, terrific, etc.). Frequently applied to misbehaving children (terrible twos is attested by 1949), then to adults. By 1913, colloquially, terrible had the sense of "very bad; extremely incompetent."

1795, in reference to Jacobins during the French Revolution, from French terroriste; see terror + -ist, and compare terrorism. Originally of state intimidation and government coercion by methods of terror; by 1866 it was used in English in reference to extremist revolutionaries in Russia who attempted to demoralize the government by terror. In recent use by 1944, especially in reference to Jewish tactics against the British in Palestine:

"Every man, woman and child of the Jewish community must do his or her best to bring this terrorism to an end." [Winston Churchill, from statement in House of Commons, Nov. 17, 1944, in response to the assassination of Lord Moyne by Jewish terrorists]

The term now usually refers to a member of a clandestine or expatriate organization aiming to coerce an established government by acts of violence against it or its subjects. [OED, 1989]

In U.S. West Coast newspapers in 1945 it was frequently in reference to attacks on Japanese returning from internment camps. Also sometimes in 19c., "an alarmist." The tendency of one party's terrorist to be another's guerrilla or freedom fighter was noted in reference to the British action in Cyprus (1956) and the war in Rhodesia (1973). The word terrorist has been applied, at least retroactively, to the Maquis resistance in occupied France in World War II (as in in the "Spectator," Oct. 20, 1979). Terrorizer is attested from 1892.