A “bag of bones” is an idiomatic expression used to describe a person who is very thin, almost to the point of emaciation, and whose bones are often visible through the skin. Idiomatic expressions are metaphorical – an exceptionally thin person isn’t actually a bag of bones, for instance, but looks like one. Their meaning is figurative, not literal, and English sayings like this rarely translate well into other languages. This is why, when hiring editors, writers or translators, astute employers insist that candidates have expertise in the idiomatic use of the language.
Idioms generally are categorized as either transparent or opaque. “Bag of bones” would generally be categorized as a transparent idiom. These are idioms which, when translated literally into another language, could likely be decoded by someone who understands only the individual words used, even if the idiom is unknown. A person's cultural and historical knowledge can play a role in how transparent a particular idiom may be.
Many idioms that are constructed as similes, including “as” or “like,” are transparent because the comparison is explicit. Examples of such similes are “skinny as a rail,” or “mad as a hatter.” This isn't a universal rule, though. Someone who's "crazy like a fox" is very clever and subtle.
Opaque idioms are less clear, and those who know only the meanings of their component words are usually unable to decode their idiomatic meanings. “Kick the bucket” and “break the ice” are examples of opaque idioms. “Bag of bones,” on the other hand, could fairly easily be figured out in a literal translation, especially in context: “You’re nothing but a bag of bones! When was your last meal?”
Idioms are considered to be more cultural in nature than linguistic, especially in the case of opaque idioms for which there’s often no understandable literal translation. There are idioms in many languages that euphemize dying, for example, that are analogous to “kick the bucket,” such as “put the spoon down” (Latvian) or “eat dandelions by the root” (French). In each of these cases, though, including the appropriate foreign-language idiom in the translation is more dependent upon knowledge of the culture than of a language’s various words and the grammatical rules for piecing them together.
English is sometimes said to be the language with the most idioms and idiomatic expressions. This is one of the reasons it’s so difficult to learn as a second language. With respect to “bag of bones,” for example, most English speakers would provide another idiom — “skinny” — as part of the definition. The use of “skinny” as a word meaning “resembling skin” was first noted in the late 16th century, while its idiomatic meaning was first recorded in the early 17th. In modern usage, though, the original meaning has all but been forgotten, and the word’s only acknowledged meanings are idiomatic.