Code used by persons who claim to be participating in vigorous, solitary outdoor activities to escape stress when in fact they are carrying on hot and heavy clandestine affairs in foreign countries (preferably located in South America).
On Saturday, Jenna tried six times to get in touch with her boyfriend, Mike, but he had already told us he was hiking the Appalachian Trail this weekend so we knew his phone would be off.
To be unfaithful to one's partner e.g. in an extramarital affair. Named after South Carolina Governor Mark Sanford, who was said to be hiking the Appalachian Trail, when actually he was visiting his mistress in Argentina.
"My husband has a lot of "meetings" and "conferences" to attend to lately. I think he might be hiking the Appalachian trail."