1630s (earlier crudle, 1580s), "to thicken, cause to congeal," frequentative of curd (v.) "to make into curd" (late 14c.; see curd). Of blood, in figurative sense "to inspire horror" from c.1600. Related: Curdled (1590); curdling (c.1700, almost always with reference to blood, in the figurative sense).