In the course of tracking down some of the more obscure vocabulary in Blood Meridian (obscure, unless you’re a Scots-descended horseman living in what is now the American Southwest), I ran across the interesting word scantlin or scantling (“I got busted in the head with a scantlin,” chap. III, p. 32 in the Modern Library edition). It refers to a timber used for framing a house or a ship, like a 2×4, and is often used in the plural. It has several other older senses, reaching back to its derivation from scantillon, jumping from the French échantillon, with senses of “a sample” or “a measuring rod.” But of course the confusion with scant is all too easy, and one proceeds with caution in tracing its etymology.
Slear (“Climbing up through ocotillo and pricklypear where the rocks trembled and sleared in the sun…”, chap. V, p. 62) does not show up in my printed references, but there are online uses of slearing as a industrial process performed on coiled metals. Perhaps a portmanteau of shear and slit? And is this the sense that Cormac McCarthy had in mind?