The Collaborative International Dictionary of English v.0.48:

Lay shaft \Lay shaft\, or Layshaft \Lay"shaft`\, n. (Mach.)
   A secondary shaft, as in a sliding change gear for an
   automobile; a cam shaft operated by a two-to-one gear in an
   internal-combustion engine. It is generally a shaft moving
   more or less independently of the other parts of a machine,
   as, in some marine engines, a shaft, driven by a small
   auxiliary engine, for independently operating the valves of
   the main engine to insure uniform motion.
   [Webster 1913 Suppl.]