Fey. Fey usually means "being in unnaturally high spirits, or unreal, enchanted," as in "elves and other fey creatures." In times past someone who suddenly acted so lighthearted was thought to be on the point of death, fey deriving from the Anglo-Saxon faege, "on the verge of death."
Fifth wheel. Looking at the fifth wheel of wagons and carriages, many people thought it had no function, but this wheel, or circular plate, which was attached to the upper side of the front axle and never touched the ground, supported the vehicle's body when it made a turn. Ignorance prevailed, however, and the expression fifth wheel came to mean "a useless or needless person or thing in any enterprise."
To get a rise out of someone. These words first applied to fish rising to the bait. Writers on the art of angling popularized the word rise in this sense three hundred years ago, and the metaphor from fly-fishing became standard English. Just as the fish rises to the bait and is caught, the person who rises to the lure of a practical joke becomes the butt of it. From its original meaning of raising a laugh at someone's expense, the expression has been extended to include the idea of attracting attention in general -- getting a rise out of a sales prospect, etc.
Penthouse. Over the years folk etymology made penthouse out of pentice, the former word sounding more familiar to English ears even if pentice was correct. The pentice was a kind of "lean-to" attached to another building, usually a church, the word akin to "appendix." It took several centuries for penthouse, first recorded in early 1500s, to become the luxurious separate apartment or dwelling on a roof of a building that it generally is today.
Round robin. The round robin was originally a petition, its signatures arranged in a circular form to disguise the order of signing. Most probably it takes its name from the ruban rond, "round ribbon," in 17th-century France, where government officials devised a method of signing their petitions of grievances on ribbons that were attached to the documents in a circular form. In that way, no signer could be accused of signing the document first and risk having his head chopped off for instigating trouble. Ruban rond later became round robin in English and the custom continued in the British navy, where petitions of grievances were signed as if the signatures were spokes of a wheel radiating form it's hub. Today a round robin usually means a sports tournament where all of the contestants play each other at least once and losing a match doesn't result in immediate elimination.