Noun
bag of bones (plural bags of bones)
(idiomatic) A skinny, malnourished person or animal.
I woke up, a bag of bones. Literally. They had gathered up my bones and put them in a bag and thrown the bag into a river. Derek Landy
If I don't go to the gym and work out, I look like a bag of bones. I go three times a week usually and it's nearly all weights work to help with definition. Jonas Armstrong