Etymology
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piezo- 

word-forming element meaning "pressure," from Greek piezein "to press tight, squeeze," from PIE *pisedyo- "to sit upon" (source also of Sanskrit pidayati "presses, oppresses"), from *pi "on," short for *epi (see epi-) + root *sed- (1) "to sit." First in piezometer (1820), an instrument for ascertaining or testing pressure. It was in common use in word formation from c. 1900.

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urge (v.)

1550s, from Latin urgere "to press hard, push forward, force, drive, compel, stimulate," perhaps [de Vaan] from a PIE root *urgh- "to tie, bind" (source also of Lithuanian veržti "tie, fasten, squeeze," vargas "need, distress," vergas "slave;" Old Church Slavonic vragu "enemy;" Gothic wrikan "persecute," Old English wrecan "drive, hunt, pursue"), via a notion of "to weigh down on," hence "to insist, impel." The other possibility is that the PIE root is *ureg- "to follow a track." Related: Urged; urging.

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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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cram (v.)

Old English crammian "press something into something else," from Proto-Germanic *kramm- (source also of Old High German krimman "to press, pinch," Old Norse kremja "to squeeze, pinch"), from extended form of PIE root *ger- "to gather."

From early 14c. as "fill with more than can be conveniently contained." Meaning "study intensely for an exam in a short time" (with a view to passing the test, not real learning) is attested by 1803, transitive as well as reflexive, originally British student slang. Related: Crammed; cramming; crammer.

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*angh- 

Proto-Indo-European root meaning "tight, painfully constricted, painful."

It forms all or part of: agnail; anger; angina; angry; angst; anguish; anxious; hangnail; quinsy.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit amhu- "narrow," amhah "anguish;" Armenian anjuk "narrow;" Lithuanian ankštas "narrow;" Greek ankhein "to squeeze," ankhone "a strangling;" Latin angere "to throttle, torment;" Old Irish cum-ang "straitness, want;" Old English enge "narrow, painful," Old Norse angra "to grieve, vex, distress," Gothic aggwus "narrow."

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intrude (v.)
early 15c., in an ecclesiastical sense, "take possession of (a prebend) not rightfully one's own," a back-formation from intrusion, or else from Latin intrudere "to thrust in, force in," from in- "in" (from PIE root *en "in") + trudere "to thrust, push," from PIE *treud- "to press, push, squeeze" (see threat).

From 1560s in a physical sense of "thrust in" (transitive or intransitive); meaning "enter unbidden and without welcome" is from 1570s; that of "thrust or bring in without necessity or right" is from 1580s. Related: Intruded; intruding.
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express (v.1)

late 14c., "represent in visual arts; put into words," from Old French espresser, expresser "press, squeeze out; speak one's mind" (Modern French exprimer), Medieval Latin expressare, frequentative of Latin exprimere "represent, describe, portray, imitate, translate," literally "to press out" (source also of Italian espresso); the sense evolution here perhaps is via an intermediary sense such as "clay, etc., that under pressure takes the form of an image," from ex "out" (see ex-) + pressare "to press, push," from Latin premere "to press, hold fast, cover, crowd, compress" (from PIE root *per- (4) "to strike"). Related: Expressed; expresses; expressing; expressible.

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clam (n.)

bivalve mollusk, c. 1500 (in clam-shell), originally Scottish, apparently a particular use of Middle English clam "pincers, vice, clamp" (late 14c.), from Old English clamm "bond, fetter, grip, grasp," from Proto-Germanic *klam- "to press or squeeze together" (source also of Old High German klamma "cramp, fetter, constriction," German Klamm "a constriction"), possibly from a PIE *glem- or *glom- "contain, embrace" (see glebe).

If this is right then the original reference is to the shell. Clam-chowder attested from 1822. To be happy as a clam is from 1833, but the earliest uses do not elaborate on the notion behind it, unless it be self-containment.

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strain (v.)

c. 1300, "tie, bind, fasten, gird," from present participle stem of Old French estreindre "bind tightly, clasp, squeeze," from Latin stringere (2) "draw tight, bind tight, compress, press together," from PIE root *streig- "to stroke, rub, press" (source also of Lithuanian strėgti "congeal, freeze, become stiff;" Greek strangein "twist;" Old High German strician "mends nets;" Old English streccian "to stretch;" German stramm, Dutch stram "stiff").

From late 14c. as "tighten; make taut," also "exert oneself; overexert (a body part)," Sense of "press through a filter, put (a liquid) through a strainer" is from early 14c. (implied in strainer); that of "to stress beyond measure, carry too far, make a forced interpretation of" is from mid-15c. Related: Strained; straining.

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bouncer (n.)

1762, "one who bounces," agent noun from bounce (v.), which originally meant "to thump, hit." Given various specific senses in 19c., such as "boaster, bully, braggart" (1833); also "large example of its kind" (1842); "enforcer of order in a bar or saloon" (1865, American English, originally colloquial).

Now it so happened that the brakeman was what is known, in the language of the road, as a "bouncer." That is, he was a hybrid combining the qualities of a brakeman and a bruiser, and was frequently called into requisition by the conductor to take the dirty work of ejecting tramps off of his hands. ["Staats," "A Tight Squeeze," 1879]
"The Bouncer" is merely the English "chucker out". When liberty verges on license and gaiety on wanton delirium, the Bouncer selects the gayest of the gay, and — bounces him! [London Daily News, July 26, 1883]
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