lagging

1. Insulation used to prevent heat diffusion, as from a steam pipe.
2. A wooden frame built especially to support the sides of an arch until the keystone is positioned.

[From LAG².]

adjective

Not progressing and developing as fast as others, as in economic and social aspects: backward, underdeveloped, undeveloped. See precede/follow.

1. Thermal insulation for pipes, tanks, ducts, etc.; sometimes block insulation, pre-shaped to conform to the curved surface.
2. The planking, consisting of narrow strips, extending from one rib of the centering of an arch or vault to another; provides direct support for the voussoirs until the arch or vault is closed in.
3. Boards which are joined, side by side, lining an excavation.
4. Horizontal members between soldier piles.
5. Wood strips that cover a wall.

Retarded or diminished movement of the affected side of the chest in pulmonary tuberculosis.

The noun has one meaning:

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