aposiopesis (n.)

rhetorical artifice wherein the speaker suddenly breaks off in the middle of a sentence, 1570s, from Latin, from Greek aposiopesis "a becoming silent," also "rhetorical figure of breaking off," from aposiopan "become silent," from apo "off, away" (see apo-) + siope "silence," from PIE root *swī- "to be silent." Related: Aposiopetic.