General:
* Emaciated: This is a strong word suggesting extreme thinness due to starvation or illness.
* Skeletal: This emphasizes the visibility of bones under the skin.
* Wasted: This implies a loss of flesh and strength.
* Underweight: This is a more clinical term.
* Cachectic: This describes a state of extreme weakness and emaciation.
* Gaunt: This emphasizes the angularity and sharpness of the features.
More specific:
* Anorexic: This specifically refers to someone with an eating disorder.
* Malnourished: This describes someone lacking essential nutrients.
* Dehydrated: This refers to someone lacking water.
* Starved: This emphasizes the cause of the thinness.
Figurative:
* Frail: This emphasizes weakness.
* Lanky: This suggests a tall, thin build.
* Skinny: This is a more informal term.
The best word to use depends on the specific context and the level of formality you want to achieve.