1
general::
Phrase(s): Suppose I don’t? [and] Supposing I don’t?
Inf. And what will happen if I don’t? (Said as a sort of threat. Not usually with question intonation.) • Bill: You’d better get yourself over to the main office. Tom: Suppose I don’t? • Father: You simply must do better in school. Tom: Supposing I don’t? Father: Your clothing and personal belongings will be placed on the curb for the garbage pickup, and we will have the locks changed. Next question.
McGrawhill's American Idioms And Phrasal Verbs