not supposed to
1. Intended, expected, or believed to do something. This new software is supposed to make things a lot more efficient, but I find it so confusing that everything is taking me twice as long! We're supposed to arrive around 3 PM, assuming our flights aren't delayed.
2. Required or obligated to do something. I was supposed to be home an hour ago—my parents are going to kill me! Please don't distract him, he's supposed to be cleaning his room.
3. Allowed or permitted to do something. (Chiefly used in questions and negative constructions.) You're not supposed to go in there! Are you sure you're supposed to be in here?
See also: supposed
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
(It's) not supposed to.and (Someone's) not supposed to.
a phrase indicating that someone or something is not meant to do something. (Often with a person's name or a pronoun as a subject. See the examples.) Fred: This little piece keeps falling off. Clerk: It's not supposed to. Bill: Tom just called from Detroit and says he's coming back tomorrow. Mary: That's funny. He's not supposed to.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.