spin·na·ker (spĭn'ə-kər) n. A large triangular sail set on a spar that swings out opposite the mainsail, used on yachts when running before the wind.
[Perhaps ultimately from Sphinx, name of the first yacht to use such a sail, or spin, to move rapidly (variant of Scots spene, to run before the wind; see spindrift).]
"large triangular sail," 1866, either a derivative of spin in the sense of "go rapidly" or a corrupt pronunciation of Sphinx, which was the name of the first yacht known to carry this type of sail.