When a person is described as leggy, it means they have very long legs. If your little sister is extremely tall and leggy for her age, you might suggest that she try out for the school basketball team.
When a lovely woman is described as leggy, it's usually in an admiring way: "I felt like New York was full of leggy models that weekend." If you describe a plant as leggy, it's more likely to be critical, meaning something closer to "scrawny" or "spindly." When the daisies in your garden get too leggy, you'll have to stake them or even prune them so they can grow back fuller and healthier.