1. To get up on and sit into the mount (saddle) of a horse. Don't feel bad—I've been riding horses for a few years, and I still have trouble mounting up. OK, campers—pack up your gear and mount up! It's time to head out.

2. To increase in size, amount, scope, or intensity. As the pressures of managing the company continued mounting up, Martin found it harder and harder to cope. If these costs keep mounting up, I don't know how we'll be able to keep the business running.

3. To set or affix something at a higher position (on something else). In this usage, a noun or pronoun can be used between "mount" and "up." I was going to mount the new TV up on the wall, but I actually prefer it on the table where it is now. The soldier mounted up the machine gun on the rampart in preparation of the invading forces.