ship-lap n. (a) a form of joint in carpentry made by halving (see quots. and halving n.1 2, and cf. half-lap at lap n.3 2b); (b) boards interlocked by rebates, used esp. for cladding.
1854 A. E. Baker Gloss. Northamptonshire Words, Ship-lap, a carpenter's term for a mode of uniting the end of one piece of wood to the side of another, at right angles, by a bevil-shaped bearing on the upper edge.
1939 W. Faulkner Wild Palms 15 The flimsy walls (they were not even tongue-and-groove..but were of ship-lap).
1977 Cornish Times 19 Aug. 13/2 (advt.) Our..plumbing, shiplap claddings and drainage systems are always in stock.