Joke Editor's Dictionary #9:

Ultimatum - n. - a last demand before resorting to concessions (Ambrose Bierce)

Vegetarian - n. - an old Indian word meaning "lousy hunter" (Andy Rooney)

Whom - pronoun - a word that was invented to make everyone sound like a butler (Calvin Trillin)

X-ray - n. - an image often used by persons interested in becoming bone models (The Onion Book of Known Knowledge)

Yoga - n. - a Sanskrit word that means "Heal your back without insurance" (Norman K.)

Zucchini - n. - a vegetable that can be baked, boiled, fried or steamed before kids refuse to eat it.