Etymology
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nudge (v.)

"to push slightly with the elbow," 1670s, perhaps from Scandinavian (compare Norwegian nugge, nyggje "to jostle, rub;" Icelandic nugga "to rub, massage"). Figurative sense of "give a hint or signal to," as by a covert touch, is by 1831. Related: Nudged; nudging.

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semaphore (n.)

"mechanical apparatus for signaling to distant points," 1814, from French sémaphore, etymologically, "a bearer of signals," ultimately from Greek sēma "sign, signal" (see semantic) + phoros "bearer," from pherein "to carry" (from PIE root *bher- (1) "to carry"). Related: Semaphoric (1808); semaphorist.

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indicator (n.)
1660s, "that which indicates or points out," from Late Latin indicator, agent noun from indicare "to point out, show" (see indication). As a finger muscle, from 1690s. As a steam-cylinder's pressure gauge, 1839. As a device on a motor vehicle to signal intention to change direction, 1932.
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beckon (v.)

Old English gebecnian (West Saxon beacnian) "to make a mute sign, signal by a nod or gesture," from Proto-Germanic *bauknjan (source also of Old Saxon boknian, Old High German bouhnen), from PIE root *bha- (1) "to shine" (compare beacon). Related: Beckoned; beckoning. The noun is attested from 1718, from the verb.

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flashlight (n.)
also flash-light, 1886, "on-and-off signal light in a light-house, etc.," from flash (v.) + light (n.). As the word for a photographer's light-emitting preparation, 1892 (flash-lamp in this sense is by 1890). From 1905 as as a handheld, pocket-sized electric illumination device, the American English word for what the British might call an electric torch.
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countersign (n.)

"a military watchword, a signal given to a soldier on guard, with orders to let no one pass who does not first give that signal," 1590s, from French contresigne, from contre- "against" (see contra-) + signe "sign" (see sign (n.)).

COUNTERSIGN. A watchword used by military bodies as a precaution against an enemy or enemies. The countersign may be changed at any moment, or any number of times, but is usually altered each twenty-four hours. It is given primarily to commanders of guards, and outposts and their sentries, to reconnoitring and visiting patrols, and to the field and regimental officer of the day. All others desiring to pass through the lines must first be supplied with the countersign, which is thus a guard against spies, strangers, and surprise. ["New International Encyclopaedia," 1906]
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etaoin shrdlu 
1931, journalism slang, the sequence of characters you get if you sweep your finger down the two left-hand columns of Linotype keys, which is what typesetters did when they bungled a line and had to start it over. It was a signal to cut out the sentence, but sometimes it slipped past harried compositors and ended up in print.
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vibration (n.)

1650s, from Latin vibrationem (nominative vibratio) "a shaking, a brandishing," noun of action from past participle stem of vibrare "set in tremulous motion" (from PIE root *weip- "to turn, vacillate, tremble ecstatically"). Meaning "intuitive signal about a person or thing" was popular late 1960s, but has been recorded as far back as 1899. Related: Vibrational.

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audible (adj.)

"able to be heard," 1520s, from French audible and directly from Medieval Latin audibilis "that may be heard," from Latin audire "to hear" (from PIE root *au- "to perceive"). Related: Audibly; audibility; audibleness. As a noun, "thing capable of being heard," from 1610s. Football sense of "signal called at the line of scrimmage to change the play called in the huddle" is by 1958.  

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semiotic (adj.)

1620s, "of symptoms, relating to signs of diseases," from Latinized form of Greek sēmeiōtikos "significant, portending, worth marking," also "observant of signs," adjective form of sēmeiosis "indication," from sēmeioun "to signal, to interpret a sign," from sēmeion "a sign, mark, token," from sēma "sign" (see semantic). Its use in linguistics and psychology, "of or pertaining to the use of signs," is by 1923. Related: Semiotical (1580s).

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