From Longman Dictionary of Contemporary Englishset off phrasal verb1 LEAVE A PLACEto start to go somewhere I’ll set off early to avoid the traffic. for Jerry and I set off on foot for the beach.2 set something ↔ offSTART something/MAKE something START to make something start happening, especially when you do not intend to do so News that the claims might be true set off widespread panic. Hong Kong’s stock market fell, setting off a global financial crisis.RegisterIn written English, people often say that something triggers a particular reaction or event, rather than sets it off, because it sounds more formal:This could trigger a global financial crisis.3 set something ↔ off to make an alarm start ringing Smoke from a cigarette will not normally set off a smoke alarm.4 set something ↔ offSCBEXPLODE to make a bomb explode, or cause an explosion Any movement could have set off the bomb.5 set something ↔ offSUIT/LOOK GOOD TOGETHER if a piece of clothing, colour, decoration etc sets something off, it makes it look attractive The blue sundress set off her long blonde hair. 6 set somebody offSTART something/MAKE something START to make someone start laughing, crying, or talking about something Don’t mention what happened – you’ll only set her off again.7 set something off against taxPET to officially record the money you have spent on something connected with your job, in order to reduce the amount of tax you have to pay Some expenses can be set off against tax. → set→ See Verb table
Examples from the Corpus
set off• Nobody knows what set the riot off.• Police say radio signals were probably used to set the bomb off.• The alarm is so sensitive that the slightest movement will set it off.• The group set off a bomb outside a crowded cafe in Izmir last September.• The army's invasion set off a major international crisis.• The splitting of an atom sets off an explosive chain reaction.• In 1992, tidal waves set off by a strong earthquake killed around 2000 people.• Smoke alarms can be set off by smoke from cigarettes, cooking, and fireplaces.• If we set off early in the morning we should reach the coast before dark.• We set off for Brighton in good spirits.• It's a lovely dress, and a brightly coloured silk scarf will set it off perfectly.• Someone accidentally set off the fire alarm.• The brass rail sets off the wooden panelling very nicely.set for• They agreed they were quite lucky to escape the fire and set off for a foreign country.• When the rain stops I set off for a walk in the cool, after-rain air.• Gregorio and I set off for London.• Thomson, eager to experience as many of these opportunities as possible, set off for Penang in 1862.• We got some torches together and set off for the graveyard.• Wilcock set off for the New World.• She set off for the second floor, but there was no police officer there.• Refreshed, and filled once again with energy we set off for the second time that day. 