Bunch wings are exactly that! A bunch of fibres, or two or more bunches tied in as wings! "Bunched and split" is also commonly specified. Hackle fibres, quill fibres or hair in the appropriate colour may be used. Many carefully prepared and tied in matched wing slips are reduced to a bunch anyway after a few casts in the bushes or a few good takes from fish. Many people prefer to tie in a bunch wing to start with, they are a little easier to prepare and tie them in than matched feather wing slips for instance. The fish rarely seem to care, in fact sometimes they will take a scruffy tattered old-looking fly in preference to a pristine fly fresh from the box!
The "Bunched and Split" technique involves cutting a slip usually from a primary wing feather, it should be four times as wide as the finished wing. Manipulate the slip to align the tips, then fold it in half lengthwise, and then again. This gives you your normal wing width. This folded slip is tied on and pulled up to near vertical. Now take your thread through the centre of the slip, going from back to front, you can ease the slip apart with a dubbing needle if required. This splits the folded fibres in two. These will split even more in use and end up looking like two bunches anyway, but flies dressed thus are often deadly flies for dead drifting.