I can give you so many examples of when words hurt. Think back to school or other situations where you were teased, seen kids teased OR been the one to do the teasing. It hurt, it sucks and I bet as an adult you don't use those teasing words anymore.
Add this to the list of things you no longer say or think, expand your vocabulary to come up with ways to substitute for this word.
March 3 is End the R-word Day. Help Spread the Word to raise the consciousness of society about the dehumanizing and hurtful effects of the word "retard(ed)" and encourage people to pledge to stop using the R-word! Pledge your support at http://www.r-word.org