Textbook describes something that perfectly matches a standard example or definition, like it came straight from a teaching manual. When a doctor sees a textbook case of the flu, the patient shows every classic symptom exactly as medical books describe. A coach might praise a textbook tackle in football, meaning the player executed it with perfect form. When meteorologists see a textbook hurricane developing, its structure matches exactly what the science predicts.

Textbooks present the clearest, most standard examples of concepts, so when something in real life matches those examples perfectly, we call it textbook.

Sometimes people use the word negatively. A textbook mistake means making an error so obvious and predictable that it's probably warned about in training materials. A textbook disaster means something went wrong in the most predictable way possible, with all the classic warning signs that someone should have noticed.

The phrase works because textbooks exist to show you the pure, ideal version of things. Real life usually gets messier and more complicated, so when something unfolds exactly as the books describe, it's actually pretty remarkable.