sego lilyn. A western North American bulbous plant (Calochortus nuttallii) having large white to lavender cup-shaped flowers.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.se?go lil?y (?si go?) n. a plant, Calochortus nuttallii, of the western U.S., having bell-shaped flowers and an edible root. [