To rile someone is to make them really angry — not a mild annoyance, but hot and bothered. It is informal, and you hear it a lot in American speech: Don't rile him. It often has the feeling of poking at a person until they snap. You can also rile a group up, as if you were stirring a crowd. Though actually, it began as a country way of saying roil, which first meant to shake water until it goes cloudy; the make-someone-mad meaning came after. In careful writing, annoy or anger is safer; rile sounds spoken, a bit rough, sometimes almost playful.