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).
Online Etymology Dictionary, © 2010 Douglas Harper
The condition of a fluid thickened almost to a solid by inspissation.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.