Etymology
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coercion (n.)

early 15c., cohercioun, "compulsion, forcible constraint," from Old French cohercion (Modern French coercion), from Medieval Latin coercionem, from Latin coerctionem, earlier coercitionem, noun of action from past-participle stem of coercere "to control, restrain" (see coerce).

It defies the usual pattern where Middle English -cion reverts to Latin type and becomes -tion. Specific sense in reference to government by force, ostensibly to suppress disorder, emerged from 19c. British policies in Ireland. "As the word has had, in later times, a bad flavour, suggesting the application of force as a remedy, or its employment against the general sense of the community, it is now usually avoided by those who approve of the action in question" [OED].

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enforcement (n.)

late 15c., "constraint, compulsion," from Old French enforcement "strengthening, fortification; rape; compulsion, coercion;" from enforcier; see enforce + -ment. Meaning "compelling of obedience to a law, etc." is from 1680s.

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duress (n.)

early 14c., "harsh or severe treatment," from Old French duresse, durece, from Latin duritia "hardness," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." For Old French -esse, compare fortress. Sense of "coercion, compulsion" is from early 15c.; in law, "actual or apprehended physical constraint so great as to amount to coercion" (early 15c.).

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voluntarism (n.)

1838, "theory or principal of using voluntary action rather than coercion (in politics, religion, etc.), from voluntary + -ism. (Voluntaryism in the religious sense, as opposed to establishmentarianism, is recorded from 1835.) In philosophy, "theory that the will is the basic principle," 1896, from German Voluntarismus (Tönnies, 1883).

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-tion 

syllable formed when the word-forming element -ion (from Latin -io) is fixed to a base or to another suffix ending in -t or -te. In Middle English, in words via Old French, it often was -cion (in coercion and suspicion, however, the -c- belongs to the base).

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threat (n.)

Old English þreat "crowd, troop," also "oppression, coercion, menace," related to þreotan "to trouble, weary," from Proto-Germanic *thrautam (source also of Dutch verdrieten, German verdrießen "to vex"), from PIE *treud- "to push, press squeeze" (source also of Latin trudere "to press, thrust," Old Church Slavonic trudu "oppression," Middle Irish trott "quarrel, conflict," Middle Welsh cythrud "torture, torment, afflict"). Sense of "conditional declaration of hostile intention" was in Old English.

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coerce (v.)

mid-15c., cohercen, "restrain or constrain by force of law or authority," from Old French cohercier, from Latin coercere "to control, restrain, shut up together," from assimilated form of com- "together" (see co-) + arcere "to enclose, confine, contain, ward off," from PIE *ark- "to hold, contain, guard" (see arcane). The unetymological -h- was perhaps by influence of cohere. Related: Coerced; coercing. No record of the word between late 15c. and mid-17c.; its reappearance 1650s is perhaps a back-formation from coercion.

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compulsion (n.)

early 15c., "coercion, application of force (to someone) overwhelming his preferences," from Old French compulsion, from Latin compulsionem (nominative compulsio) "a driving, urging," noun of action from past-participle stem of compellere "to drive, force together," from com "with, together" (see com-) + pellere "to drive" (from PIE root *pel- (5) "to thrust, strike, drive").

Psychological sense of "instant impulse to behave in a certain way" is from 1909 in A.A. Brill's translation of Freud's "Selected Papers on Hysteria," where German Zwangsneurose is rendered as compulsion neurosis.

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constraint (n.)

late 14c., constreinte, "distress, oppression," a sense now obsolete, from Old French constreinte "binding, constraint, compulsion" (Modern French contrainte), fem. noun from constreint, past participle of constreindre, from Vulgar Latin *constrinctus, from Latin constrictus, past participle of constringere "to bind together, tie tightly, fetter, shackle, chain," from assimilated form of com "with, together" (see con-) + stringere "to draw tight" (see strain (v.)).

Meaning "coercion, compulsion, irresistible force or its effect to restrict or compel" is from 1530s. Especially "repression of emotion or of the expression of one's thoughts or feelings" (1706).

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T 

twentieth letter of the English alphabet; in the Phoenician alphabet the corresponding sign was the 22nd and last; everything after T in the modern alphabet represents European alterations or additions. The sound has been consistent throughout its history.

In Late Latin and Old French, -t- before -e- and -i- acquired the "s" value of -c- and words appeared in both spellings (nationem/nacionem) and often passed into Middle English with a -c- (nacioun). In most of these the spelling was restored to a -t- by or in the period of early Modern English, but sorting them out took time (Edmund Coote's "English Schoole-maister" (1596) noted malicious/malitious) and a few (space, place, coercion, suspicion) resisted the restoration. 

To cross one's t's(and dot one's i's) "to be exact" is attested from 1849. Phrase to a T "exactly, with utmost exactness" is recorded from 1690s, though the exact signification remains uncertain despite much speculation. The measuring tool called a T-square (sometimes suggested as the source of this) is recorded by that name only from 1785. The T-cell (1970) so called because they are derived from the thymus. As a medieval numeral, T represented 160. A T was formerly branded on the hand of a convicted thief.

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