Etymology
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alarm (v.)

1580s, "call to arms for defense," from alarm (n.) or from French alarmer (16c.), from the noun in French. Meaning "surprise with apprehension of danger" is from 1650s. Related: Alarmed; alarming.

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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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alarmed (adj.)
"disturbed by prospects of peril," 1640s, past-participle adjective from alarm (v.).
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alarmist (n.)
"one addicted to sounding alarms," 1793, from alarm (n.) + -ist.
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alarum (n.)
obsolete and poetic spelling of alarm (n.).
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alarmingly (adv.)
1787, from alarming, present-participle adjective from alarm (v.), + -ly (2).
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*ar- 
also arə-, Proto-Indo-European root meaning "to fit together."

It forms all or part of: adorn; alarm; aristarchy; aristo-; aristocracy; arm (n.1) "upper limb of the body;" arm (n.2) "weapon;" armada; armadillo; armament; armature; armilla; armistice; armoire; armor; armory; army; art (n.) "skill as a result of learning or practice;" arthralgia; arthritis; arthro-; arthropod; arthroscopy; article; articulate; artifact; artifice; artisan; artist; coordination; disarm; gendarme; harmony; inert; inertia; inordinate; ordain; order; ordinal; ordinance; ordinary; ordinate; ordnance; ornament; ornate; primordial; subordinate; suborn.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit irmah "arm," rtih "manner, mode;" Armenian arnam "make," armukn "elbow;" Greek arti "just," artios "complete, suitable," artizein "to prepare," arthron "a joint;" Latin ars (stem art-) "art, skill, craft," armus "shoulder," artus "joint," arma "weapons;" Old Prussian irmo "arm;" German art "manner, mode."
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clock-radio (n.)

"combined bedside radio and alarm clock which can be set to turn on the radio instead of sounding the alarm," 1946, from clock (n.1) + radio (n.).

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tocsin (n.)

"alarm bell," 1580s, from French toquassen "an alarm bell, the ringing of an alarm bell" (late 14c.), from Old Provençal tocasenh, from tocar "to strike" (from Vulgar Latin *toccare "strike a bell;" see touch (v.)) + senh "bell, bell note," from Late Latin signum "bell, ringing of a bell," in Latin "identifying mark, sign" (see sign (n.)). The current English spelling is from 1794, adopted from modern French.

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buffalo (v.)
"alarm, overawe," 1900, from buffalo (n.). Probably from the animals' tendency to mass panic. Related: Buffaloed; buffaloing.
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