The word you are looking for is vulgar. It is vulgar to use the word turd. Any word can be a swear word if you swear it; example: “That person is a complete Plutoid.”
@MissA: Ok so two things were being silly (sense of humour required, I just like to fill out my sentences) but “well, you can polish a turd” is a saying, for example when referring to an ugly building that has been redecorated, one may say “well you can polish a turd, but its still a turd” – turd as a metaphor was a thrust of the question, though its use a noun was also part of my quandary. I’m sorry that wasn’t made clear.
I just heard my grandson say “Oh fecal doo doo” as a swear word. I think his mother taught him to say it because I don’t allow swear words in my house.