For this one the etymology is useful.
It's a Latin word, formed with the prefix con- (meaning 'with') plus the verb descendere (meaning 'go down, descend'), and together meaning to lower oneself metaphorically in status by associating with inferiors.
This is Classical Roman culture, remember; it featured a rigidly enforced vertical social hierarchy, with rich nobles at the top, slaves at the bottom, and everybody else in between.
In Modern English, with a somewhat different social system — and extremely different social values — the word has come to mean not so much to treat everyone alike — which is now a good thing, after all — but rather to be perceived to be making an effort to treat everyone alike, especially one's presumed "inferiors".
Since this is a matter of interpretation, often by those who feel condescended to (note the Passive of an intransitive verb with a transitivizing preposition, like insist on), condescension is easy to complain about, but very hard to distinguish from personal dislike.