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general::
Phrase(s): *on a diet
trying to lose weight by eating less food or specific foods. (*Typically: be ~; go ~; put someone ~; stay ~.) • I didn’t eat any cake because I’m on a diet. • I’m getting too heavy. I’ll have to go on a diet. • I have a lot of trouble staying on a diet.
McGrawhill's American Idioms And Phrasal Verbs