DNC stands for Definitely Not a Crap.
A "crap" is an attractive girl. When you see them you say, "oh, crap."
So when the term DNC is used, you are referring to an unattractive woman.
Josh - "Hey, Chris you see that chick?"
Chris - "Yea man, shes a DNC"
Carl: "Not true. Regarding the 13th, 14th and 15th Amendments, the DNC attempted to block their passage and keep slavery legal but had enough dissenters join in with the Republicans to get the 2/3 majority each Amendment needed to pass. 12 days after the ratification of the 13th Amendment, several high-ranking DNC members formed the Ku Klux Klan in Pulaski, Tennessee to harass, intimidate, beat and kill freed slaves."
Chad: "Dude, that's hypocritical of them. I'm leaving the DNC and going Independent."