The Collaborative International Dictionary of English v.0.48:

Lagging \Lag"ging\, n.
   1. (Mach.) The clothing (esp., an outer, wooden covering), as
      of a steam cylinder, applied to prevent the radiation of
      heat; a covering of lags; -- called also deading and
      cleading.
      [1913 Webster]

   2. Lags, collectively; narrow planks extending from one rib
      to another in the centering of arches.
      [1913 Webster]