thicken

verb (used with or without object)

1. to make or become thick or thicker.
2. to make or grow more intense, profound, intricate, or complex: The plot thickens.

verb

1. to make or become thick or thickerthicken the soup by adding flour
2. (intr) to become more involvedthe plot thickened

v.early 15c. (transitive), 1590s (intransitive), from thick + -en (1). Related: Thickened; thickening. An earlier verb was Old English þiccian.