Etymology
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havoc (n.)
early 15c., from the expression cry havoc "give the signal to pillage" (Anglo-French crier havok, late 14c.). Havok, the signal to soldiers to seize plunder, is from Old French havot "pillaging, looting" (in crier havot), which is related to haver "to seize, grasp," hef "hook," probably from a Germanic source (see hawk (n.)), or from Latin habere "to have, possess." General sense of "devastation" first recorded late 15c.
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sensor (n.)

"device giving a signal about some physical activity," 1947, from a shortened form of sensory (q.v.) or an agent noun in Latin form from sense (v.).

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feedback (n.)
1920, in the electronics sense, "the return of a fraction of an output signal to the input of an earlier stage," from verbal phrase, from feed (v.) + back (adv.). Transferred use, "information about the results of a process" is attested by 1955.
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flag (v.2)

1875, "place a flag on or over," from flag (n.1). Meaning "designate as someone who will not be served more liquor," by 1980s, probably from use of flags to signal trains, etc., to halt, which led to a verb meaning "inform by means of signal flags" (1856, American English). Meaning "to mark so as to be easily found" is from 1934 (originally by means of paper tabs on files). Related: Flagged; flagger; flagging.

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route (v.)

1890, of a railroad ticket, "mark for use on a certain route," from route (n.). The meaning "direct (an electrical signal, phone call, etc.) over a particular circuit or to a particular location" is by 1948. Related: Routed; routing; routeing (1881).

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beck (n.)
late 14c., "nod or other mute signal intended to express desire or command," a noun use from Middle English bekken (v.), variant of becnan "to beckon" (see beckon). Transferred sense of "slightest indication of will" is from late 15c.
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highball (n.)
type of alcoholic drink, 1898, probably from ball "drink of whiskey;" high (adj.) because it is served in a tall glass. The word also was in use around the same time as railway jargon for the signal to proceed (originally by lifting a ball).
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responder (n.)

"one who or that which responds or replies," by 1834 of persons; 1867 of devices (telegraphy); agent noun from respond (v.). Meaning "device which automatically retransmits a pulse or signal" is by 1945.

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flare (n.)
"a giving off of a bright, unsteady light," 1814, from flare (v.). This led to the sense of "signal fire" (1883). The astronomy sense is from 1937. Meaning "a gradual widening or spreading" is from 1910; hence flares "flared trousers" (1964).
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curfew (n.)

early 14c., curfeu, "evening signal, ringing of a bell at a fixed hour" as a signal to extinguish fires and lights, from Anglo-French coeverfu (late 13c.), from Old French cuevrefeu, literally "cover fire" (Modern French couvre-feu), from cuevre, imperative of covrir "to cover" (see cover (v.)) + feu "fire" (see focus (n.)). Related: Curfew-bell (early 14c.).

The medieval practice of ringing a bell (usually at 8 or 9 p.m.) as an order to bank the hearths and prepare for sleep was to prevent conflagrations from untended fires. The modern extended sense of "periodic restriction of movement" had evolved by 1800s.

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