Shotgun·An exclamation made upon sight of a vehicle in which a passenger will travel, designating the seat as proprietary of the exclaimer for the duration of an upcoming trip.
Antidisestablishmentarianism·The movement to prevent revoking the Church of England's status as the official church.
Rickrolling·Tricking people into listening to Rick Astley's Never Gonna Give You Up instead of the content that they expected to see.
Affluenza·A feeling of dissatisfaction, anxiety, etc, caused by the dogged and ongoing pursuit of more.
Just·I was just asked to define it by a non-native English speaker. I can think of no other English word that can take on so many different meanings.
Autoinfanticide·The act of going back in time and killing yourself as an infant and associated with the grandfather paradox of time travel.
To clothesline somebody·To hit someone in the neck area with your inner arm, usually when they are moving at a high rate of speed, causing them to fall down (video of clotheslining in description).
Cafeteria Christian·A person who professes Christian adherence, but who selectively chooses which doctrines to follow.
Congenial·Pleasing or liked on account of having qualities or interests that are similar to one's own.
Streaker·A specific type of exhibitionist that interrupts public events, especially sports matches, by invading them while nude.