Like a twig — and a twig is one of those thin little woody stems that grow out from a branch. So 'twiggy' describes something very thin, dry-looking and easily snapped. You use it two ways. About plants and places, it means full of twigs or made of twigs: a twiggy bush in winter, a twiggy nest, twiggy little branches with no leaves left. About people, it means uncomfortably thin — twiggy arms, twiggy legs — and this is not a compliment; it suggests fragile and underfed rather than slim and healthy. It is an everyday describing word, not a formal one, and it appears far more in writing about nature than in conversation.