I've done my best to explain terms as I've been using them, but it will be useful to have a concise resource for the hiker lingo that gets slung around so often.
A.T.: short for Appalachian Trail
Blue-blazing: Taking a side trail (typically marked with blue blazes) that is not part of the official AT route. There are blue blazed trails spreading off the AT everywhere, but blue-blazing specifically refers to the practice of skipping part of the official route by using a side trail. A good way for elitist thru-hikers to look down their nose at other hikers.
HYOH: short for "hike your own hike". A trail mantra of sorts that reminds us that all hikers have a different reason for being out there and that no one can say you're doing it wrong. Serves as a suitable defense against almost any trail practice.
eg: "I just saw a thru-hiker carrying an electronic keyboard. He's never going to make it!"
"You should talk; the only thing you brought to eat is peanut butter."
"HYOH I guess."
Katahdin: the mountain that marks the northern terminus of the AT in Maine. The ultimate destination for northbound hikers, and the starting point for southbounders.
MEGA: an adjective or noun, depending on whom you ask, referring to a southbound thru hike. Maine to Georgia = ME-GA. No one really uses this term in speech, so I suspect it is just to add variety to prose. Also known as SOBO.
Nero: see Zero below. A day in which a hiker completes some, but not very many, trail miles. Like a zero, useful for recovery, but still allows the hiker to feel a little accomplishment during the day.
NOBO: a northbound thru-hiker. Most people attempt to thru-hike in this direction, starting in March-May, and ending no later than October. They head from Springer Mountain in Georgia to Mt. Katahdin in Maine.
Pink-blazing: the act of trying to catch up with a female hiker. This is made possible by shelter registries, in which most thru-hikers jot down some notes about what they've been up to as they pass through. Pink-blazing is a not-uncommon pastime as there is a significant gender imbalance on the trail.
Privy: an outhouse along the trail, typically found near shelters. These vary wildly in quality.
PUDs: pointless ups and downs. A disdainful term for useless undulations of the trail as it follows every ridgeline and notch the Appalachian Mountain range has to offer.
Slackpacking: The act of completing a section of the trail with a reduced load, i.e. a daypack. This can be accomplished by arranging for someone to transport your full pack to your destination or by throwing your pack ahead to enjoy a few feet of unencumbered walking. Some hikers maintain that no honest thru-hike includes slackpacking, but I would direct these people to "HYOH" (above).
SOBO: a southbound thru-hiker (me!). They travel from Maine to Georgia, and almost always start later than their NOBO counterparts. A much smaller crowd than northbounders. See also MEGA.
Springer Mountain: the mountain that marks the southern terminus of the
AT in Georgia. The ultimate destination for southbound hikers, and the starting
point for northbounders.
Thru-hike: the act of completing the entire length of the Appalachian Trail within a year. Now, there are as many definitions of a thru-hike as there are AT hikers, but I just went with the simplest definition I could think of. Some people say that you haven't truly completed a thru-hike if you skip any part of the trail (see "blue-blazing" or "yellow-blazing"), and I belong to that school of thought.
Trail angel: a perpetrator of trail magic (see below). Regarded highly by thru-hikers, trail angels are extremely helpful for restoring morale and keeping many hikers going on the trail.
Thru-hiker: See "thru-hike" above. This definition is up to interpretation as well, but my definition is as follows: someone who is in the process of attempting to complete the entire AT in one year, OR someone who has completed a thru-hike. In my opinion, when someone suspends a thru-hike attempt to go home and rest, they leave their thru-hiker status in a state of suspended animation, waiting for them at the trailhead, but with a (roughly) one-year expiration date.
Trail magic: acts of goodwill and generosity found along the trail. Typical examples are scout troops cooking hot dogs for hikers at a trailhead or a random stranger hanging up a six-pack of beer in the woods.
Trail name: a moniker given to just about every thru-hiker on the trail. It acts like a second identity, as most hikers don't go by their actual name once on the trail. A trail name can be chosen by the hiker, but most often it is chosen by fellow thru-hikers. Stories behind trail names are almost universally interesting and make for good conversation.
White blaze: a 2 inch x 6 inch swath of paint used to denote the entire course of the Appalachian Trail. You'll typically find these on trees when they're available, but above treeline you'll find white blazes on rocks or other visible places.
Yellow-blazing: a term coined because of the color of road lines on the blacktop, this is the practice of shaving off miles from an AT hike by following roadways, typically in a car.
Yogi: the act of getting tourists or day hikers to give you food, while still making them think it was their idea. Named after the bear. I never mastered this but it would have been a very useful skill.
Zero: a day in which a hiker completes zero trail miles. Helpful for restoring spirits, filling up bellies, and rejuvenating ailing body parts.