If someone is bow-legged, their legs curve out at the knees, so there is a gap between the knees when the feet are together. The legs make a shape a bit like a bow — the curved thing you use to shoot an arrow, or the curve of a rainbow. You can also write it as one word, bowlegged. It is an ordinary, slightly blunt description of a body; doctors may use more careful language, but everyday English is happy with bow-legged. People are born this way, or it can come from childhood illness or from work that stresses the legs. It is not an insult in itself, though saying it of someone who is standing right there can feel unkind, as body comments often do. The opposite picture is knock-kneed, where the knees fall in, not out.