'blood-curdling' originates from English as a compound of 'blood' + 'curdle' + the present participle suffix '-ing'; here 'curdle' carries the literal sense 'to clot' or 'to turn to curds', applied figuratively to blood.

'blood-curdling' developed from the phrase 'to curdle the blood' (used figuratively in Early Modern English) into the adjective form 'blood-curdling' recorded in later English usage to describe terrifying things or sounds.

Initially it referred more literally to the idea of blood 'curdling' (clotting) as a physical reaction to shock; over time it became a figurative expression meaning 'extremely frightening' or 'horrifying'.