is a contraction of display (to unfold; Latin, dis-plico A splay window is one in a V-shape, the external opening being very wide, to admit as much light as possible, but the inner opening being very small. A splay-foot is a foot displayed or turned outward. A splay-mouth is a wide mouth, like that of a clown.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Splay from Fact Monster: