Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1400, "density, thickness, compactness," from Latin spissitudo, from spissus "thick, dense, compact, close" (source of Italian spesso, Spanish espeso, Old French espes, French épais).
spissitude spis·si·tude (spĭs'ĭ-tōōd', -tyōōd')
The condition of a fluid thickened almost to a solid by inspissation.