Injurious things should be avoided. They cause pain, harm, and — yes — injuries. Handle with care!
If you know what an injury is, you have a head start on learning injurious. Anything injurious is dangerous. At a construction site, almost anything could be injurious, if you don't know what you're doing (or even if you do). Falling off a ladder could be very injurious. Even crossing the street can have injurious effects if you're hit by a car. If this word turns up in your diary a dozen times, you're probably in the hospital.