The Latin prefix con- means "with," and the Latin word for descend, descendere, means "descend" or "down," so the word condescending probably developed to describe someone who looked down on others. Condescending behavior is, not surprisingly, itself looked down upon.  The word condescending came to Middle English from Old French condescendre, from the Latin  condescendere

The adjective "condescending" was first used in the mid 1600s.  Specifically, the Oxford English Dictionary identifies the earliest known use in the writing of Roger Boyle, 1st Earl of Orrery, in 1655.  The noun form, "condescension," also emerged around the same time, with its first recorded use in 1642. The adverb "condescendingly" appeared even earlier, in 1653. 

Examples of "condescending" used in a sentence:  
"I didn't like his smug, condescending tone"
"Some faculty members seem to express a condescending, at times almost disdainful, attitude." 
"I sensed him smiling down in a condescending way and ushering me back into the building and onto the stage, tears and all."
"Your answers to me were patronizing and condescending." 
"His condescending tone was as annoying as the words."
"Jackson always picked some condescending nickname for her beaus."
"Your condescending tone is making me feel patronized and insulted."
"Don't use that condescending attitude with me!"

33 minutes ago, Bill W said:

 Condescending behavior is, not surprisingly, itself looked down upon. 

As @Bill W has pointed out we tend to be condescending towards people who are condescending.

Now I'm going to go a bit weird here. Should there be a particular word for being condescending towards a person who is condescending?

This could expand to should we have a word for being a bully to bullies, an artist who paints artists or a person who comments on comments. Nah that's getting far too weird.

Just the facts:

CONDESCENDING Definition & Meaning - Merriam-Webster

CONDESCENDING Synonyms: 149 Similar and Opposite Words | Merriam-Webster Thesaurus