What is the X in X-mas?
rhetorical artifice wherein the speaker suddenly breaks off in the middle of a sentence, 1570s, from Latin, from Greek aposiopesis "a becoming silent," also as a rhetorical figure, from apo- (see apo-) + siope "silence."
(Greek: "becoming silent"), a speaker's deliberate failure to complete a sentence. Aposiopesis usually indicates speechless rage or exasperation, as in "Why, you . . .," and sometimes implies vague threats as in, "Why, I'll . . . ." The listener is expected to complete the sentence in his mind. In ancient Greek rhetoric, the aposiopesis occasionally takes the form of a pause before a change of subject or a digression.