Etymology
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cue (n.1)

"words spoken at the end of a speech in a play that are the signal for an answering speech," 1550s, of uncertain origin. By one theory it is a spelling out of Q, the letter, which was used 16c., 17c. in stage plays to indicate actors' entrances and was explained at the time as an abbreviation of Latin quando "when" (from PIE root *kwo-, stem of relative and interrogative pronouns) or a similar Latin adverb. Shakespeare's printed texts have it as both Q and cue. Cue as a name for "the letter Q" is attested from 1755.

Transferred to music by 1880. Figurative sense of "sign or hint to speak or act" is from 1560s. The television reader's cue-card is attested by 1948.

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reaction (n.)

"action in resistance or response to another action or power," 1640s, from re- "back, again, anew" + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), a noun of action formed in Late Latin from the past-participle stem of Latin reagere "react," from re- "back" + agere "to do, perform."

Originally a word in physics and dynamics. In chemistry, "mutual or reciprocal action of chemical agents upon each other," by 1836. The general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.

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gavel (n.)
"small mallet used by presiding officers at meetings," 1805, American English, of unknown origin; perhaps connected with German dialectal gaffel "brotherhood, friendly society," from Middle High German gaffel "society, guild," related to Old English gafol "tribute," giefan "to give" (from PIE root *ghabh- "to give or receive"). But in some sources gavel also is identified as a type of mason's tool, in which case the extended meaning may be via freemasonry. As a verb, by 1887, from the noun. Old English had tabule "wooden hammer struck as a signal for assembly among monks," an extended sense of table (n.).
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ensign (n.)
early 15c., "a token, sign, symbol; badge of office, mark or insignia of authority or rank;" also "battle flag, flag or banner of a ship or troop of soldiers," via Scottish, from Old French enseigne (12c.) "mark, symbol, signal; flag, standard, pennant," from Latin insignia (plural); see insignia, which is a doublet of this word. As the word for the soldier who carries the flag, 1510s. U.S. Navy sense of "commissioned officer of the lowest rank" is from 1862. French navy had rank of enseigne de vaisseau at least since early 18c. Until 1871 one of the lowest grades of commissioned officers in a British army infantry regiment, also a rank in the American Revolutionary army.
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pipe (v.)

Old English pipian "to play on a pipe" or similar instrument, from Latin pipare "to peep, chirp," of imitative origin (see pipe (n.1)). Compare Dutch pijpen, German pfeifen.

From 1590s, of birds, "to chirp, warble, whistle, sing." Meaning "convey through pipes" is by 1887. Related: Piped; piping. Piping hot is in Chaucer, a reference to hissing of food in a frying pan.

To pipe up (early 15c.) originally meant "to begin to play" (on a musical instrument); sense of "to speak out" is from 1856. Pipe down "be quiet" is from 1900, probably a reversal of this, but earlier (and concurrently) in nautical jargon it was a bo'sun's whistle signal to dismiss the men from duty (1833); pipe in the nautical sense of "to call by the pipe or whistle" is by 1706.

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doom (n.)

Middle English doome, from Old English dom "a law, statute, decree; administration of justice, judgment; justice, equity, righteousness," from Proto-Germanic *domaz (source also of Old Saxon and Old Frisian dom, Old Norse domr, Old High German tuom "judgment, decree," Gothic doms "discernment, distinction"), perhaps from PIE root *dhe- "to set, place, put, do" (source also of Sanskrit dhaman- "law," Greek themis "law," Lithuanian domė "attention").

Originally in a neutral sense but sometimes also "a decision determining fate or fortune, irrevocable destiny." A book of laws in Old English was a dombec. Modern adverse sense of "fate, ruin, destruction" begins early 14c. and is general after c. 1600, from doomsday and the finality of the Christian Judgment. Crack of doom is the last trump, the signal for the dissolution of all things.

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bell (n.)

"hollow metallic instrument which rings when struck," Old English belle, which has cognates in Middle Dutch belle, Middle Low German belle but is not found elsewhere in Germanic (except as a borrowing); apparently from PIE root *bhel- (4) "to sound, roar" (compare Old English bellan "to roar," and see bellow).

As a division of daily time aboard a ship, by 1804, from its being marked by bells struck every half hour. Statistical bell curve is by 1920, said to have been coined was coined 1870s in French. Of glasses in the shape of a bell from 1640s. Bell pepper is from 1707, so called for its shape. Bell, book, and candle is a reference to a form of excommunication (the bells were rung out of order and all together to signify the loss of grace and order in the soul of the excommunicated).

To ring a bell "awaken a memory" (1934) is perhaps a reference to Pavlovian experiments; it also was a signal to summon a servant (1782).

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charge (n.)

c. 1200, "a load, a weight," from Old French charge "load, burden; imposition," from chargier "to load, to burden," from Late Latin carricare "to load a wagon or cart," from Latin carrus "two-wheeled wagon" (see car). A doublet of cargo.

Meaning "responsibility, burden" is from mid-14c. (as in take charge, late 14c.; in charge, 1510s), which progressed to "pecuniary burden, cost, burden of expense" (mid-15c.), and then to "price demanded for service or goods" (1510s). Meaning "anything committed to another's custody, care, or management" is from 1520s.

Legal sense of "accusation" is late 15c.; earlier "injunction, order" (late 14c.). Meaning "address delivered by a judge to a jury at the close of a trial" is from 1680s. Electrical sense is from 1767. Slang meaning "thrill, kick" (American English) is from 1951. Meaning "quantity of powder required for one discharge of a firearm" is from 1650s. Military meaning "impetuous attack upon an enemy" is from 1560s; as an order or signal to make such an attack, 1640s.

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thirty (adj., n.)

"1 more than twenty-nine, twice fifteen; the number which is one more than twenty-nine; a symbol representing this number;" late 14c. metathesis of thritti, from Old English þritig, from þri, þreo "three" (see three) + -tig "group of ten" (see -ty (1)). Similar formation in Old Frisian thritich, Old Saxon thritig, Dutch dertig, Old High German drizzug, German dreissig.

The Thirty Years' War (1842) was a general European religious and dynastic power struggle, furiously destructive, waged 1618-48, mainly on German soil. The symbol -30- as printer and telegrapher's code to indicate the last sheet or line of copy or a dispatch is recorded from 1895. In 20c. jargon of newspaper journalism, it came to be a traditional sign-off signal and slang word for "the end."

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turn (n.)
c. 1200, "action of rotating," from Anglo-French tourn (Old French torn, tour), from Latin tornus "turning lathe;" also partly from turn (v.). Meaning "an act of turning, a single revolution or part of a revolution" is attested from late 15c. Sense of "place of bending" (in a road, river, etc.) is recorded from early 15c. Meaning "beginning of a period of time" is attested from 1853 (as in turn-of-the-century, from 1921 as an adjectival phrase).

Sense of "act of good will" is recorded from c. 1300. Meaning "spell of work" is from late 14c.; that of "an individual's time for action, when these go around in succession" is recorded from late 14c. The automatic automobile turn-signal is from 1915. Turn-sick "dizzy," is attested from early 15c. Phrase done to a turn (1780) suggests meat roasted on a spit. The turn of the screw (1796) is the additional twist to tighten its hold, sometimes with reference to torture by thumbscrews.
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