This expression was often used in David Willis' comics by Joyce, an overly religious character uncomfortable with her own and other people's sexuality.
It can be used as a humorous reference to any form of extramarital sex, especially to mock people with attitudes similar to Joyce's.
A male person who is so incredibly hot, he can melt the panty hose right off most female people. Term originated in the late 1980s early 1990s when people actually wore panty hose.