From Longman Dictionary of Contemporary EnglishI guess so/notI guess so/notspoken used to agree or disagree with a statement or question‘You’re one lucky guy.’ ‘I guess so.’‘I don’t really have any choice, do I?’ ‘I guess not.’ →guess
Examples from the Corpus
I guess so/not• "She wasn't happy?" "I guess not."• Dope, is it? I guess so.