also "puny"

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Note added at 18 mins (2007-08-18 08:49:25 GMT)
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bony: very thin especially from disease or hunger or cold; "emaciated bony hands"; "a nightmare population of gaunt men and skeletal boys"; "eyes were haggard and cavernous"; "small pinched faces"; "kept life in his wasted frame only by grim concentration"
wordnet.princeton.edu/perl/webwn

(used especially of persons) of inferior size
wordnet.princeton.edu/perl/webwn

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Note added at 26 mins (2007-08-18 08:57:28 GMT)
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Dear Asker,

Bellow is an excerpt from a text I have translated recently. The word "puny" is used in this piece of interview by a native speaker:

Jack Kelly .. was determined all the children would have beautiful bodies. And there was little puny Grace with her asthma and .. he thought that this was the runt of the litter. .. Ma Kelly .. was bored with the little puny one.

I hope this can help you decide.

a skinny person is extremely thin, often in a way that you find unattractive.(informal)

This is a commonly used phrase for someone who is so thin that he or she looks to be near death.

This term is the most common ( in use) of informal that I now of. Puny means small as well so it might cause confusuion. A bag of bones is a noun not an adjective. Skinny is also correct. Lanky is a bit more informal.

Cheers

there are many different terms to describe a thin guy, and "skinny" does it in a humiliating way I think.