Emaciated means painfully thin from hunger or illness. Bones showing where they should not.
It is a serious clinical word rather than a description of build, which is why it belongs to medicine, famine reporting and descriptions of neglected animals, and using it about somebody merely slim is wrong. It carries real alarm. The verb emaciate exists and is almost never used. Gaunt and skeletal are its neighbours, and gaunt suggests a face while emaciated suggests a whole body.
1adjectiveemaciatedAbnormally thin and weak from hunger or illness.
“The dogs were emaciated when they were found.”