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. Source
Antidisestablishmentarianism·The movement to prevent revoking the Church of England's status as the official church. Source
Rickrolling·Tricking people into listening to Rick Astley's Never Gonna Give You Up instead of the content that they expected to see. Source
Widow-maker·A dead branch caught precariously high in a tree which may fall on a person below. Source
Affluenza·A feeling of dissatisfaction, anxiety, etc, caused by the dogged and ongoing pursuit of more. Source
pearl-clutching·To act overly shocked as to publicly display your strong aversion to something. Source
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. Source
Autoinfanticide·The act of going back in time and killing yourself as an infant and associated with the grandfather paradox of time travel. Source
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). Source
to fail upwards·To get promoted into a higher career position despite failing at the current job. Source
Cafeteria Christian·A person who professes Christian adherence, but who selectively chooses which doctrines to follow. Source
Congenial·Pleasing or liked on account of having qualities or interests that are similar to one's own. Source
Streaker·A specific type of exhibitionist that interrupts public events, especially sports matches, by invading them while nude. Source