To improve the performance or appearance of something by customizing it with new components or decorations. A noun or pronoun can be used between "mack" and "out." I bought a car for less than $2,000, then macked it out with speakers, new tires, and a sick paint job. The company urges people to bring in their old, beat-up computers and let their team of engineers mack out the old machines, rather than throw them away and buy brand new ones.