Duty=Stupid

Like, what I would like to think, most people we are teaching our kiddos not to say certain words. For instance, we think that some words my not be considered swear words, they can still be offensive and hurtful. We try to expand our vocabulary and try to find words that are worth using to describe our happiness, frustration, or sadness.
Zoey keeps saying the word DUTY. Only she puts emphasis on the "U" and it sounds like DOOTY. Yesterday I caught her saying, "That duty thing!" I asked what a duty was, and with all the seriousness in the whole entire world she proceeds to enlighten me on the fact that she CAN'T say stupid, so duty will have to work. CAN YOU BELIEVE THIS??? What is a mom to say??
I looked her square in the eye and asked her if she knew what the word duty ment, like it has a bad meaning??? She said no, turned around and walked away from me. I was speechless.