"It's a disgusting thing to call someone. . . . Dirty blood, see. Common blood. It's ridiculous. Most wizards these days are half-blood anyway. If we hadn't married Muggles we'd've died out."
-- Ron Weasley while spitting up slugs (CS7)
A foul, nasty name for Muggle-born is Mudblood (lit. “dirty blood”), but that’s not a term used in polite company (CS7).
Draco Malfoy loved using the word "Mudblood,"especially when the Chamber of Secrets opened in second year. Referring to Hermione as a "filthy little Mudblood" caused Ron to come to her defense with his broken wand, rebounding a Slug-Vomiting Charm onto himself (CS6). When Harry and Ron disguised themselves as Crabbe and Goyle with the Polyjuice Potion, Draco felt comfortable in front of them, saying "Saint Potter, the Mudbloods' friend. He's another one with no proper wizard feeling, or he wouldn't go around with that jumped up Granger Mudblood" (CS12).
As part of Umbridge's Inquisitorial Squad, Draco Malfoy took ten points from Gryffindor because Hermione was a "Mudblood" (OP28).
Umbridge wrote a pamphlet as Head of the Muggleborn Registration Commission entitled "MUDBLOODS and the Dangers They Pose to a Peaceful Pure-Blood Society" (DH13)
Voldemort seemed to equate Muggles and Muggle-borns, calling his father "a Muggle and a fool. . . very like your dear mother" (GF33). Voldemort captured Charity Burbage, the Muggle Studies teacher, and tortured at Malfoy Manor for writing an "impassioned plea" for acceptance of Muggleborns. He told his followers, " She would have use all mate with Muggles . . . or, no doubt, werewolves . . . ” (DH1)
Sirius Black's mother, Walburga, used the word profusely when shrieking from her portrait: "'MUDBLOODS! SCUM! CREATURES OF DIRT!'" (OP10). When the House of Black house-elf Kreacher started ranting about the Weasleys, Harry and Hermione as "Mudbloods and traitors and scum…" Sirius picked him up by his loin cloth and threw him out the door of the room (OP6). Harry inherited Kreacher after Sirius died, and had to forbid him from calling anyone "Mudblood" ever again (DH10).
Pius Thicknesse, Minister for Magic under Voldemort, agreed with Kreacher and Mrs. Black: “If you ask me, the blood traitors are as bad as the Mudbloods" (DH13).
Severus Snape made a terrible mistake when he called Lily Evans (Harry's future mother) a "Mudblood" in front of their classmates. It ended their relationship as childhood friends (OP28,DH33). Ironically, nine-year-old Lily had worried about being Muggleborn, but child-Snape told her "It doesn't make any difference" - after all he was a Half-Blood with a Muggle father himself (DH33). As adult Headmaster of Hogwarts, he chastised the portrait of Phineas Nigellus for calling Hermione "the Mudblood," saying "Do not use that word!" (DH33).