To mass dump things, first set a dump zone with i, then once back to no menus open press d>b>d. This will allow you to designate a whole area of items by pressing enter, then drawing a box around the items with the arrow keys, and then enter again. Every item in the area will be designated for dumping. Once a dwarf with refuse hauling takes the job to dump the item, he will bring it to the dump zone. A minor yet very useful exploit is to make a one square dump zone. Dwarves will pile an unlimited amount of items in that square. For example, a use for this would be to dump all the stone in a recently excavated area directly in a room with your mason, stonecrafter, and mechanics workshop in it.
In addition, if your bookkeeper has an accurate count of the items in the fort, you can mass dump through the stockpile screen. This can be useful if you want to drop all the animal remains outside to rot away without making miasma.
Do remember that all items that have been dumped are automatically flagged as forbidden until you reclaim them. You can do this with d>b>c similar to mass dumping. D>b>f is mass forbid, and is useful to keep things you don't want a dwarf to touch from being messed with. A good example of this is dead dwarves clothing in the event of a catastrophe (like being slain by a rampaging forgotten beast that you will be walling inside the caverns.)