bulk up

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1. Size, mass, or volume, especially when very large.
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
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.
1. To cause to swell or expand.
2. To cause to cohere or form a mass.
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.
in bulk
1. Unpackaged; loose.
2. In large numbers, amounts, or volume.

[Middle English, perhaps partly alteration of bouk, belly, trunk of the body (from Old English būc) and partly from Old Norse bulki, cargo, heap; see bhel- in Indo-European roots.]

bulk up

(adverb) to increase or cause to increase in size or importance
References in periodicals archive ?
Studio Iore has it that Held managed to leave with pounds of discarded raw pigment, which he and his friend Ronald Bladen used to bulk up the impasto of their "action paintings.
Most professional swimmers train from six to eight hours a day, but that much is not required for the guy who wants to basically look better and feel better but doesn't necessarily want to bulk up.
Although their informational literature and year-end reports may tout how well the stock market's revival drove returns in 2004, pension funds didn't neglect to bulk up on real estate last year and in the years prior.