Ricochet

Meaning - Spring back;spring away from an impact (as a verb) and a glancing rebound(as a noun).

The origin of this french word is unknown. This was used as military term in earlier times to express the rebound of a projectile that strikes on a hard surface.It is said that during 18th century, field artillery, which was not,before Napolean's time,relied upon the ricochet of round shot.The term "ricochet" is now only applied, in modern rifle shooting, to the graze of a bullet that has struck short.

I came across its use in below line from The New York times "Some Muslim supporters of Mr. Obama seem to ricochet between dejection and optimism."

References: http://encyclopedia.jrank.org

Underdog

Underdog - One that is at a disadvantage.

I always thought that the “dog” bit here referred to dogfight. But as they say, “a little knowledge is a dangerous thing". So a little bird-dogging revealed that instead of canine reference, dog in underdog is actually a plank of wood. The word supposedly originates from shipbuilding where the planks of wood (called dogs) were sawn for their construction. The senior saws-man stood on top of the plank and he was the overdog. The junior had to go below the planks. And…….. no brownie points for guessing that he was called the "underdog". As simple as that! :)
Source: Wikipedia, Answers.com