Appalachian Trail hikers use lots of words that only have significance on the trail. Here's a list of the most commonly used lingo and what it means:
trail name- every hiker adopts a nickname. Sometimes you pick it yourself, and sometimes someone else names you. You might get your name because of your favorite food, a weird habit, or something silly you do. Examples of names are Monk, Rainbow Monkey, Moonshine, Blue Eyes.
AT- Appalachian Trail
thru-hiker- a person who is hiking the entire trail in one calendar year
section-hiker- a person who is hiking the trail sections at a time
day-hiker- a person who is only out for a day. Can be recognized by clean smell.
NOBO or Northbounder- a person who is hiking North.
SOBO or Southbounder- a person who is hiking South.
white blaze- a 2 inch by 6 inch swatch of white paint on trees and rocks that marks the trail.
blue blazer- a person who takes the blue-blazed (usually easier) side paths
yellow-blazer- a person who gets a ride further ahead on the trail. Named for the yellow lines on roads.
pink-blazer- a guy who is making it a point to follow a female hiker.
flip-flip or flip-flopper- a person who hikes in one direction then returns to the starting point and hikes the other direction
PUDS- pointless ups and downs
switchback- when the trail goes in a zig-zag pattern up or down a very steep mountain
register- a notebook that is kept at shelters and other places for hikers to sign their name and write something
hiker box- a place where hikers can ditch unwanted gear or food, and other hikers can take what they want
slack-pack- hiking without a full pack, which is sent ahead to be picked-up later
purist- a person who hikes past every single white blaze with a full pack.
trail angel- a person who does nice favors for hikers such as giving them food, drinks, or rides
trail magic- when something unexpected and good happens, normally in the form of free food and drinks
Yogi-ing- trying to get free food from others
cairn- a pile of rocks that marks the trail in treeless areas
zero day- hiking zero miles in one day
nero day- hiking a little more than zero miles in a day, like 2 or 3