Etymology
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impotent (adj.)
late 14c., "physically weak, enfeebled, crippled," from Old French impotent "powerless, weak, incapable of doing," from Latin imponentem (nominative impotens) "lacking control, powerless, feeble; lacking self-control," from assimilated form of in- "not, opposite of" (see in- (1)) + ponentem (nominative potens) "potent" (see potent).

Meaning "having no power to accomplish anything" is from mid-15c.; that of "completely lacking in sexual power" (of males) is from mid-15c. Middle English also had a native term for this: Cunt-beaten (mid-15c.). The figurative sense in Latin was "without self-control, headstrong, violent, ungovernable, lacking self-restraint," which sometimes is found in English (OED cites examples from Spenser, Massinger, Dryden, and Pope). Related: Impotently.
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impotence (n.)
early 15c., "physical weakness," also "poverty," from Old French impotence "weakness" (13c.), from Latin impotentia "lack of control or power," from impotentem "lacking control, powerless" (see impotent). In reference to a complete want of (male) sexual potency, from c. 1500. The figurative senses of the word in Latin were "violence, fury, unbridled passion," via the notion of "want of self-restraining power," and these sometimes were used in English. Related: Impotency.
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*poti- 

Proto-Indo-European root meaning "powerful; lord." 

It forms all or part of: bashaw; compos mentis; despot; hospodar; host (n.1) "person who receives guests;" idempotent; impotent; omnipotent; pasha; plenipotentiary; posse; possess; possible; potence; potency; potent; potentate; potential; potentiate; potentiometer; power; totipotent

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit patih "master, husband;" Greek posis, Lithuanian patis "husband;" Latin potis "powerful, able, capable; possible." 

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invalid (adj.2)
"of no legal force," 1630s, from special use of Latin invalidus "not strong, infirm, impotent, feeble, inadequate," from in- "not" (see in- (1)) + validus "strong" (from PIE root *wal- "to be strong").
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invalid (adj.1)
"not strong, infirm," also "infirm from sickness, disease, or injury", 1640s, from Latin invalidus "not strong, infirm, impotent, feeble, inadequate," from in- "not" (see in- (1)) + validus "strong" (from PIE root *wal- "to be strong"). With pronunciation from French invalide (16c.).
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terrorism (n.)

1795, in specific sense of "government intimidation during the Reign of Terror in France" (March 1793-July 1794), from French terrorisme, noted in English by 1795 as a coinage of the Revolution, from Latin terror "great fear, dread, alarm, panic; object of fear, cause of alarm; terrible news," from PIE root *tres- "to tremble" (see terrible).

If the basis of a popular government in peacetime is virtue, its basis in a time of revolution is virtue and terror — virtue, without which terror would be barbaric; and terror, without which virtue would be impotent. [Robespierre, speech in French National Convention, 1794]

General sense of "systematic use of terror as a policy" is first recorded in English 1798 (in reference to the Irish Rebellion of that year). At one time, a word for a certain kind of mass-destruction terrorism was dynamitism (1883); and during World War I frightfulness (translating German Schrecklichkeit) was used in Britain for "deliberate policy of terrorizing enemy non-combatants."

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pussy (n.2)

slang for "female pudenda," by 1879, but probably older; perhaps from Old Norse puss "pocket, pouch" (compare Low German puse "vulva"), or perhaps instead from the cat word (see pussy (n.1)) on the notion of "soft, warm, furry thing;" compare French le chat, which also has a double meaning, feline and genital. Earlier uses are difficult to distinguish from pussy (n.1), e.g.:

The word pussie is now used of a woman [Philip Stubbes, "The Anatomie of Abuses," 1583]

And songs such as "Puss in a Corner" (1690, attributed to D'Urfey) clearly play on the double sense of the word for ribald effect. But the absence of pussy in Grose and other early slang works argues against the vaginal sense being generally known before late 19c., as does its frequent use as a term of endearment in mainstream literature, as in:

"What do you think, pussy?" said her father to Eva. [Harriet Beecher Stowe, "Uncle Tom's Cabin," 1852]

Pussy-whipped "hen-pecked" is attested by 1956 (Middle English had cunt-beaten "impotent," in reference to a man, mid-15c.).

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