From Samuel Beckett’s first novel, Murphy:
He broke into a sweat, lost all his yellow, his heart pounded, the garret spun round, he could not speak. When he could he said, in a voice new to Ticklepenny:
‘Have fire in this garret before night or—’
He stopped because he could not go on. It was an aposiopesis of the purest kind. Ticklepenny supplied the missing consequences in various versions, each one more painful than any that Murphy could have specified, terrifying taken all together.
Aposiopesis /ˌæpəsaɪəˈpiːsɪs/ “APuh-SYuh-PEE-sis” (audio) is a fancy rhetorical term for a familiar act: an abrupt breaking off of a thought, mid-sentence, often because of overwhelming emotion. Aposiopeses is the plural form, aposiopetic the adjectival.
The word entered English in the 1570s from Latin, which took it from the Greek aposiōpēsis, from aposiōpan, “to become silent”; siōpē means “silence”. In writing it is signalled by an em dash, as in Beckett, above, or by an ellipsis, as used by P.G. Wodehouse in The Adventures of Sally:
“So…” said Mr. Carmyle, becoming articulate, and allowed an impressive aposiopesis to take the place of the rest of the speech. A cold fury had gripped him.
Sometimes the speaker continues with a separate thought (“Why I oughta— C’mere you little—”); the critical thing is that one thought is interrupted because the speaker is unable or unwilling to finish the sentence.
The Simpsons has a famous recurring example: “Why you little—!”