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set sth off

(CAUSE) phrasal verb with set
 
 
/set/ verb (present tense setting, past tense and past participle set)

Definition

C2 to cause an activity or event, often a series of events, to begin or happen: The court's initial verdict in the police officers' trial set off serious riots.Starting and beginningStarting again to cause a loud noise or explosion, such as that made by a bomb or an alarm (= a warning sound) to begin or happen: Terrorists set off a bomb in the city centre. Somebody set the alarm off on my car.Exploding and erupting
(Definition of set sth off (CAUSE) from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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