bulk (b?lk)n.1. Size, mass, or volume, especially when very large.2. a. A distinct mass or portion of matter, especially a large one: the dark bulk of buildings against the sky.b. The body of a human, especially when large or muscular.3. The major portion or greater part: “The great bulk of necessary work can never be anything but painful” (Bertrand Russell).4. See fiber.5. Thickness of paper or cardboard in relation to weight.6. A ship’s cargo.v. bulked, bulk·ing, bulks v.intr.1. To be or appear to be massive in terms of size, volume, or importance; loom: Safety considerations bulked large during development of the new spacecraft.2. To grow or increase in size or importance.3. To cohere or form a mass: Certain paper bulks well.v.tr.1. To cause to swell or expand.2. To cause to cohere or form a mass.adj. Being large in mass, quantity, or volume: a bulk buy; a bulk mailing.Phrasal Verb: bulk up To gain weight by gaining muscle: dietary supplements that helped the weightlifters bulk up.Idiom: in bulk1. Unpackaged; loose.2. In large numbers, amounts, or volume.[Middle English, perhaps partly alteration of bouk, belly, trunk of the body (from Old English b?c) and partly from Old Norse bulki, cargo, heap; see bhel- in Indo-European roots.]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.