By extension of this, someone who is bone-thin, usually unattractively so, whether they are naturally that thin or not.
Usually accompanied by chicken legs and a lack of booty in both sexes, a lack of muscle tone in males, and a lack of breasts and hips and curves in females.
Also see Twiggy, legendary 1960s supermodel who idealized this androgynous, starving waif look.
This little twig is my granddaughter? Aren't you feeding her?! She looks like a swizzle stick!