on a diet

*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.
See also: on
References in classic literature ?
The doctor had put him on a diet, and he had to be satisfied with a small hare dressed with a dozen young and tender spring chickens.