To stack; make s.t. higher; to add another level; to be stacked on top of each other [mag-].
The -un imp. focuses on a bunch of things being stacked on top of each other. The -an imp. focuses on adding s.t. on to s.t. The pa--un imp. focuses on s.t. definite being stacked on top of s.t. else.
Add some more side planks to your boat.
Don't just leave those computers stacked on top of each other lest they get destroyed.