the word "thicken" is a transparent formation from the germanic root for density. the adjective "thick," from old english þicce, has a deep proto-germanic ancestry (þekuz) related to concepts of density and viscosity. the addition of the old english verbal suffix -en (-nian) creates a causative or inchoative verb, directly meaning "to make or become thick." this straightforward morphological process, where a property is transformed into an action, is common in english and germanic languages. the modern figurative use, as in a plot thickening, logically extends from the physical process of a liquid becoming more dense and opaque, thus less clear.

Structure: thick (dense) + en (verb suffix) thick (from old english þicce, meaning dense or not thin; germanic origin). en (a verb-forming suffix from old english -nian, used to form verbs meaning "to become or make").