A euphemism for a mysterious disappearance as a result of illicit activities.
Disappear for a few days and need an explanation? "Sweetie, I was just 'hiking the Appalachian Trail!'"
Boss asking for a note on why you were out for a four-hour lunch with his wife? Yep, you guess it - you were out hiking the Appalachian Trail!
See also Sanford, soon-to-be-former Governor of SC.