The Collaborative International Dictionary of English v.0.48:

Lag \Lag\, v. i. [imp. & p. p. Lagged; p. pr. & vb. n.
   Lagging.]
   To walk or more slowly; to stay or fall behind; to linger or
   loiter. "I shall not lag behind." --Milton.

   Syn: To loiter; linger; saunter; delay; be tardy.
        [1913 Webster]

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]

WordNet (r) 3.0 (2006):

lagging
    n 1: used to wrap around pipes or boilers or laid in attics to
         prevent loss of heat