Inadvisable means not recommended or unwise to do. When something is inadvisable, it means that doing it would likely lead to problems, even if it's not absolutely forbidden.

If you're feeling sick, it would be inadvisable to stay up late playing video games instead of resting. Your parents might not forbid it, but they'd probably warn you that it's a bad idea. When a weather forecast says swimming is inadvisable due to rough waves, the beach isn't closed, but sensible people stay out of the water.

The word suggests that while you could technically do something, people with good judgment wouldn't. It's inadvisable to spend your entire allowance on candy, inadvisable to ignore a strange noise your bicycle is making, and inadvisable to wait until the night before to start a big project.

Notice that inadvisable is gentler than words like “forbidden” or “dangerous.” It's advice, not a command. A teacher might say that skipping breakfast before a long field trip is inadvisable. You won't get in trouble for doing it, but you'll probably regret it when you're hungry and tired by lunchtime.