'alarmer' ultimately originates from Italian (phrase) 'all'arme' (to the arms), via Old French 'alarme' meaning 'alarm' or 'warning'. The English agent suffix '-er' was later added to form 'alarmer' (one who or that which alarms).

'all'arme' (Italian) > Old French 'alarme' > Middle English 'alarme'/'alarm' > English 'alarm' and then agent noun formation produced 'alarmer' and its plural 'alarmers'.

Initially connected with a call to arms or warning ('to the arms'), the term evolved to mean both a warning signal and, by agent‑formation, a person or device that gives an alarm; the core idea of a warning has been retained while the form broadened to include devices and people.