"bow-legged" refers to a body type in which a person's knees are naturally far apart when they are standing. Their legs are shaped like bows (bows used in archery). The humor in this is that a woman whose legs are bowed might tend to reveal her pubic region often while swimming. The humor is also helped by the fact that "swimmin" and "women" rhyme. I will admit that i chuckled when i saw the question but, to Tanja's point, i would never use this phrase in mixed company. It is coarse humor.