Aposiopesis is the term, coined by Otto Jespersen, for the rhetorical device by which the speaker or writer deliberately stops short and leaves something unexpressed, but yet obvious, to be supplied by the imagination, giving the impression that she is unwilling or unable to continue. It often portrays being overcome with passion (fear, anger, excitement) or modesty. The ellipsis or dash is used.
[n] breaking off in the middle of a sentence (as by writers of realistic conversations)