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]