RSI is short for repetitive strain injury. It is pain in the hands, arms, neck, or shoulders that comes from doing the same small movement again and again. Office workers get it from typing and using a mouse for hours; musicians, hairdressers, and factory workers get it too. It often starts small — a tight wrist, a numb finger — and grows if you keep going. Doctors use RSI as a family name for several problems, not one single illness; carpal tunnel is one kind you may hear about. People also stretch it loosely for any ache from computer work. It is a workplace and medical word, not slang. You say the three letters.