Perhaps frail and weak
Typically negative; words like 'slim' and 'thin' have a more positive connotation. Skinny means overly thin, in my point of view. Hope this helps:)
Yes, probably more so than mere "thin".
brittle
skinny
Scrawny means very thin; skinny and bony. TO BE THIN
The Latin word for "thin" is tenuis.
The pair word is 'thin': thick and thin.
Yes, puny is a connotation, and it is a negative connotation because it gives off a negative effect and makes you think negatively. :)
anorexic, stick, or gangly
The word narrow implies long and thin, as does slender.
Sometimes in this society it is used as a synonym for being thin, nd in that sense it can do, but when spoken of as the actual psychological illness, i wouldn't say it does.