1. Literally, to build and lean a pile of something against something. Investigators found that someone had stacked large crates against the emergency exit, blocking everyone's egress once the fire broke out. You can just stack those boxes against the wall over there.

2. To manipulate some situation or circumstances to make it harder for someone or something to succeed. Typically "the deck," "the cards," or "the odds" is used between "stack" and "against." Often used in passive constructions. The Olympic committed is facing criminal charges for stacking the odds against athletes from particular countries. Of course, simply by virtue of his being the boss's son, Jeremy has stacked the cards against the rest of us for an early promotion. These big corporations stack the deck against any small business that might stand in their way by hiring huge, high-level legal firms to drown them in litigation.