Something impractical is difficult or impossible to achieve. It might sound like a great idea, but it's not going to happen. It's just not practical.
Impractical ideas make no sense or just can’t be done. Heels so high you have a fish swimming in them are cool, but very impractical. Trying to drive from New York to California in four hours, which could never happen, is impractical. Something impractical is not likely to work. Practical ideas are feasible, but impractical ideas are based on wishful thinking. When someone tells you an idea is impractical, she’s saying, "Get real."