Beausage - A portmanteau blending "beauty" and "usage." American pronunciation (rhymes with "usage") and French pronunciation ("Bo-Sahj") are equally acceptable. Coined by Grant Peterson in an issue of the Rivendell Reader, and as used by Rick Smith in the 10/11/2009 episode of Yehuda Moon.
Cager - Derogatory name for a somebody driving a motor vehicle.
Ciclovia - A planned event during which major roads are closed to automotive traffic. More.
Idaho - to roll through a stop sign while riding a bike. A reference to Idaho Statutes Title 49 section 720, which makes it lawful in Idaho to, while riding a bike, slow to a "reasonable speed" and the proceed through the intersection without having to come to a full and complete stop. Critics of the Idaho law sensationalize it by exclaiming that it's legal for bikes to run stop signs.
MSH - "Magic Styrofoam Hat." Term for a bicycle helmet that implies skepticism of its ability to prevent any and all physical harm to the person wearing it.
Ninja - A scofflaw cyclist that wears dark clothing at night with no reflectors or lights. They tend to ride erratically and in a way that they suppose avoids getting in the way of automobiles. By being unable or unwilling to put lights on their bicycle, they enshroud themselves in a ninja-like shroud of invisibility, endangering themselves and everybody around them.
Bike Ninjas are the natural enemy of the Bike Pirates.
Panda - A self portrait taken with a cell phone or camera while riding your bike. The word is a reference to a girl on flickr who, while probably not inventing the on-bike self-shot, apparently single-handedly popularized it.
Pedefish - A pedestrian salmon. Usually only encountered in bike lanes. Coined by the Cap'n in this post.
Skunk stripe - A stripe of mud or dirty water down the back of a cyclist who doesn't have fenders.
Velo - a prefix meaning "relating to a bicycle." The wikipedia entry for "velo" redirects to "bicycle." One of the earliest versions of the bicycle is called a velocipede. "Velocity" is a measure of speed because bicycles are fast and so you measure a bicycle is by its speed, or velocity. You don't really have a velocity if you're walking, or even running really fast. You have to at least be on a bike in order to have a velocity.