The word that typically has a negative connotation is C "skinny". Merriam Webster dictionary defines "skinny" as "resembling skin or lacking sufficient flesh". Then, "slender" means "spare in frame or flesh" and "thin" means "having little extent from one surface to its opposite". Finally, "slight" refers to "having a slim or delicate build: not stout or massive in body". If you wanted to say that someone is thin but you would like to add a negative connotation to it, you would refer to them as "skinny". Taking into account word formation, this adjective is derived from "skin" meaning that the person referred to is so thin that you can look though their skin.