phrasal verb. If something sets a person off, it makes them angry, or makes them remember something and they start talking a lot.Click to see full answer. Similarly one may ask, what means set off against?(set something off against something) same as set against. Claims for expenses can be set off against tax. 2. (set something off against something) to compare one thing with another.Also Know, do up phrasal verb meaning? do up. From Longman Dictionary of Contemporary Englishdo up phrasal verb1 to fasten something, or to be fastened in a particular waydo something ↔ up Do up your coat or you’ll get cold. Simply so, how do you use set off in a sentence? Example Sentences We would like to set off for Paris by 10 o’clock tomorrow morning. As soon as the alarm went off it set off the dogs. The fact that he cannot remember to take his books to school really sets me off. Do not mention the football match. What is the meaning of sets off? Definition of ‘set off’ If something sets a person off, it makes them angry, or makes them remember something and they start talking a lot.