Etymology
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alert (adv.)

"on the watch," 1610s, from French alerte "vigilant" (17c.), from prepositional phrase à l'erte "on the watch," from Italian all'erta "to the height." Second element from erta "lookout, high tower," noun use of fem. of erto, past participle of ergere "raise up," from Latin erigere "raise" (see erect (adj.)).

The adjective is attested from 1712; the noun is from 1796 as "attitude of vigilance" (as in on the alert); 1803 as "a warning report." The verb is by 1864. Related: Alerted; alerting.

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alarm (n.)
late 14c., "a call to arms in the face of danger or an enemy," from Old French alarme (14c.), from Italian all'arme "to arms!" (literally "to the arms"); a contraction of phrase alle arme. Alle is itself a contraction of a "to" (from Latin ad; see ad-) + le, from Latin illas, fem. accusative plural of ille "the" (see le); with arme, from Latin arma "weapons" (including armor), literally "tools, implements (of war)," from PIE root *ar- "to fit together."

The interjection came to be used as the word for the call or warning (compare alert). Extended 16c. to "any sound to warn of danger or to arouse," and to the device that gives it. From mid-15c. as "a state of fearful surprise;" weakened sense of "apprehension, unease" is from 1833. Variant alarum (mid-15c.) is due to the rolling -r- in the vocalized form. Sometimes in early years Englished as all-arm. Alarm clock is attested from 1690s (as A Larum clock).
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*reg- 

Proto-Indo-European root meaning "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule."

It forms all or part of: abrogate; address; adroit; Alaric; alert; anorectic; anorexia; arrogant; arrogate; bishopric; correct; corvee; derecho; derogate; derogatory; Dietrich; direct; dress; eldritch; erect; ergo; Eric; Frederick; Henry; incorrigible; interregnum; interrogate; maharajah; Maratha; prerogative; prorogue; rack (n.1) "frame with bars;" rail (n.1) "horizontal bar passing from one post or support to another;" Raj; rajah; rake (n.1) "toothed tool for drawing or scraping things together;" rake (n.2) "debauchee; idle, dissolute person;" rakish; rank (adj.) "corrupt, loathsome, foul;" real (n.) "small Spanish silver coin;" realm; reck; reckless; reckon; rectangle; rectify; rectilinear; rectitude; recto; recto-; rector; rectum; regal; regent; regicide; regime; regimen; regiment; region; regular; regulate; Regulus; Reich; reign; resurgent; rex; rich; right; Risorgimento; rogation; royal; rule; sord; source; subrogate; subrogation; surge; surrogate; viceroy.

It is the hypothetical source of/evidence for its existence is provided by:

Sanskrit raj- "a king, a leader," rjyati "he stretches himself," riag "torture" (by racking); Avestan razeyeiti "directs," raštva- "directed, arranged, straight;" Persian rahst "right, correct;" Latin regere "to rule, direct, lead, govern," rex (genitive regis) "king," rectus "right, correct;" Greek oregein "to reach, extend;" Old Irish ri, Gaelic righ "a king," Gaulish -rix "a king" (in personal names, such as Vircingetorix), Old Irish rigim "to stretch out;" Gothic reiks "a leader," raihts "straight, right;" Lithuanian raižytis "to stretch oneself;" Old English rice "kingdom," -ric "king," rice "rich, powerful," riht "correct;" Gothic raihts, Old High German recht, Old Swedish reht, Old Norse rettr "correct."

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heads-up (adj.)
"clever, alert," 1926, from warning cry "heads up!" (i.e. "look up!"). As a noun, "a notification, a warning," by 1988.
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listen (n.)
"an act of listening," 1788, in on the listen "alert," from listen (v.).
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whistleblower (n.)
also whistle-blower, 1963 in the figurative sense, American English, from whistle (n.) as something sounded in an alert + agent noun from blow (v.1).
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fly (adj.)
slang, "clever, alert, wide awake," by 1811, perhaps from fly (n.) on the notion of the insect being hard to catch. Other theories, however, trace it to fledge or flash. Slang use in 1990s might be a revival or a reinvention.
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qui vive 

1726, in on the qui vive "on the alert," from French être sur le qui vive "be on the alert," from the phrase qui voulez-vous qui vive? sentinel's challenge, "whom do you wish to live?" In other words "(long) live who?" meaning "whose side are you on?" (The answer might be Vive la France, Vive le roi, etc.). From qui (from Latin qui "who") + vive, third person singular present subjunctive of vivre, from Latin vivere "to live" (see viva).

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