Wiktionary
n. (context US slang English) An intern, often female, who wears very revealing clothing in an office setting where more conservative attire is common.
Wikipedia
In American workplaces, a skintern is an informal term for a summer intern, usually female, who dresses in clothing more revealing than that which is common for the field in question.
The term is a portmanteau of skin and intern. It was first widely used about such interns working in offices of members of the U.S. House of Representatives in Washington, D.C. around the mid-2000s; Julia Allison claims it was used in that sense as early as 2000. Since then it has spread to other industries outside government and politics that also employ summer interns.
The phenomenon is sometimes a deliberate sartorial strategy, but more often is believed to result from ignorance of accepted professional dress standards. Older women in the offices where those interns work have often responded by advising them on how to dress more appropriately. Some feminists, however, have seen the term as yet another symptom of inherent sexism in the workplace, since it is so widely applied.