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).
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."
"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.