In Russian there is a jargon word "дрищ" (~ dreeshch) that usually (depending on the context) means either diarrhea or an extremely lean, usually undermuscled person, something like the one on the picture below (not necessarily undermuscled this severely). Is there an analogous word in English?
A person so underweight might be called emaciated.
emaciated adjective Abnormally thin or weak, especially because of illness or a lack of food. ‘she was so emaciated she could hardly stand’ - ODO
If the OP is looking for a colloquial equivalent, they could choose between
- superskinny–(informal) very skinny
- thin/skinny as a beanpole If you call someone a beanpole, you are criticizing them because you think that they are extremely tall and thin
- thin as a rake–(BrEng)extremely skinny or slender.
- thin as a rail–(AmEng)extremely skinny or slender
- gangly–a person, usually a boy or young man, who is very tall and thin and moves awkwardly
- scrawny–if you describe a person or animal as scrawny, you mean that they look unattractive because they are so thin.
- be skin and bones–to be extremely thin
- anorexic–suffering from or relating to anorexia
I've always liked to use the word Gaunt to describe this. The meaning is contextual but it fits pretty well here.
extremely thin and bony; haggard and drawn, as from great hunger, weariness, or torture; emaciated.
Use as an adjective: "He was tall and Gaunt."
"Skeletal" - Very thin; emaciated.
Used when someone is so thin that you can see the bones which make up their skeleton.
Example usage from the linked dictionary reference:
He is skeletal and scrawny with his minuscule bones poking through his clothes.
Since the OP is referring to a "jargon" word, I am going to avoid a regular dictionary and use the Urban Dictionary.
"waif" is an overly skinny person, often having their ribs showing. Often homeless children or super-skinny models are referred to as "waifs".
Such a person could be called a scrag. That M/W entry also references the term scrawny as noted in another answer.
If health is not an issue, I would add "beanpole":
noun | bean·pole | \ ˈbēn-ˌpōl
2 : a tall thin person
If the person is very tall and also thin then we sometimes refer to them as 'lanky' or 'gangly'.
E.g. Gosh... he's a really lanky guy **Lanky** adjective, lank·i·er, lank·i·est. ungracefully thin and rawboned; bony; gaunt: a very tall and lanky man. - [Dictionary]
If just thin, then they are usually referred to as 'stick thin' or if you are not meaning to be rude then people usually just say 'slender'.
E.g. My friend is very slender. **Slender** (of a person or part of the body) gracefully thin. - [OOD]