Aposiopesis [ap-uh-sahy-uh-pee-sis] (‘becoming silent’) is a rhetorical device wherein a sentence is deliberately broken off and left unfinished, the ending to be supplied by the imagination, giving an impression of unwillingness or inability to continue.
An example would be the threat ‘Get out, or else—!’ This device often portrays its users as overcome with passion (fear, anger, excitement) or modesty. To mark the occurrence of aposiopesis with punctuation an em dash or an ellipsis may be used.
The Daily Omnivore 