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).
1. The herb has been used for centuries to curdle milk.
2. The sauce should not boil or the egg yolk will curdle.
3. The sight made my blood curdle.
4. Their screams made my blood curdle with terror.