The name refers to pulling one's body up. Pull-ups are associated with utilizing an overhand (pronated, palms facing away) grip by many organizations This has been standardized with world record organizations such as Guinness World Records and recordholders.org. A "chin up" is named after either bringing the chin up to touch the bar, or alternatively, bringing it up and over the bar. As this is more easily done with a supinated grip, the term chin-up has been associated with doing pull-ups with an underhand (supinated, palms facing toward) grip. This new usage of chin up (which has a completely different meaning from the original one, and nothing to do with one's chin) has led some to use "pull up" in a more restricted use to refer to its opposite, pulling with a prone grip. -Wiki-
