If you are being condescending, you are looking down on someone. A 10-year-old who says to his sibling, "What do you know? You're only a 6-year-old!" is being condescending.
The Latin prefix con- means "with," and the Latin word for descend means "down," so the word condescending probably developed to describe someone who looked down on others. Condescending behavior is, not surprisingly, itself looked down upon. It's usually intended to make people feel bad about not knowing or having something, and it very often works.