The bottom of something is its underside, or lowest point. You might practice swimming by diving to the bottom of a swimming pool to retrieve pennies.
When you're hungry, you'll eat to the bottom of your ice cream bowl, and if you're looking for your car keys, you may need to dig to the very bottom of your tote bag. A thing's underside is also its bottom, so you might check out the bottom of your foot if you think you have a splinter. Informally, your bottom is also your buttocks, or your backside. It comes from the Old English botm, "ground" or "foundation."