Excessively thin is a term used to describe someone who is very underweight. This can be caused by a variety of factors, including medical conditions, eating disorders, or lifestyle choices. Two words that can be used to describe someone who is excessively thin are 'BONY' and 'SCRAWNY'. BONY is a four-letter word meaning having bones that are very prominent due to lack of flesh, while SCRAWNY is a seven-letter word meaning very thin and weak-looking.