Dictionary.com defines the word condescending as "showing or implying a usually patronizing descent from dignity or superiority, as in 'They resented the older neighbors' condescending cordiality.'"
We all know people that can be described as condescending; they're generally aloof, snobbish, and a whole lot of fake rolled into one. Many people today hate being condescended to, because it means that they're somehow less than the person doing the condescending.
You might then be interested to know that being condescending wasn't always a bad thing. An older, less-used definition of the word has condescend as a verb rather than an adjective, and defines it as "to behave as if one is conscious of descending from a superior position, rank, or dignity." A good example would be the Queen of England talking to someone of a lower social rank without appearing to talk down to that person. In that case, the Queen would have condescended to the lower rank of the other person.
There are letters from the American Civil War where Confederate* soldiers praise the ability of their superior officers to condescend to their level. Specifically where the superior officer was one of the upper classes, it was always seen as a good thing for them to have the ability to step down, as it were, from their higher station in life and speak plainly with their subordinates. It wasn't until much, much later that the act of condescension took on the negative connotation it has today.
Perhaps the alteration in meaning might be tied to changing attitudes in the United States in the Reconstruction Era, and particularly the pat-on-the-head condescension that many Americans felt from foreign-born travelers and politicians. James Russel Lowell wrote an essay, On a Certain Condescension in Foreigners, that ran in the Atlantic Monthly in 1869. Within Lowell's essay is the following line:
This shows an interesting shift, from the perception pre-Civil War of the ability of the upper classes to condescend to the lower classes as a more-or-less good thing to something that is seen as a bad quality to have."So long as we continue to be the most common-schooled and the least cultivated people in the world, I suppose we must consent to endure this condescending manner of foreigners toward us. The more friendly they mean to be the more ludicrously prominent it becomes. They can never appreciate the immense amount of silent work that has been done here, making this continent slowly fit for the abode of man, and which will demonstrate itself, let us hope, in the character of the people." (from Bartleby.com)
The things you learn, huh?