Note:  Do not rely on this information. It is very old.

Gaff

Gaff, a kind of boom used to extend the upper edge of the mizen, its foremost edge being furnished with two cheeks to embrace the after-part of the mast. A gaff topsail is a light, three- or four-sided sail on the mizen topmast, its head in the latter case extended by a small yard and its foot extending along the length of the gaff.