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bulk up somebody/somethingalso bulk somebody/something up
to make someone or something stronger or more powerful National Guard soldiers were bulking up airport security in the state. We have to be careful with his weight training not to bulk him up too much.
bulk upor bulk out
1. To increase in size or weight: The weightlifters are exercising in order to bulk out.
2. To cause someone or something to increase in size, weight, or degree: I hope all the food I'm eating doesn't bulk me out too much. All of the new employees have bulked out the company's payroll.