In stock sales, retail markets, and pretty much anything you can buy, somebody bought it before you did and paid... $X. Then they stuck a price tag on it that said something like $2X or $5X or $10,001X and sold it to you at a profit. They "marked up" the price they originally paid.
A markdown is the opposite, where a retailer says "no one's buying this stinky fish at this price, and I've got to get rid of it now or never, so I'll mark the price down until someone bites."
It also refers to the amount taken away from the final price of a security when it's sold to a dealer on the over-the-counter market. It's most often seen in the municipal bonds market, when no one wants to buy the stinky-fish debt of a random small town.
C'mon people, this bond isn't that bad. And it's on the cheap! Look at that markdown.