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1.
a diet plan that entails foregoing one's usual meal regimen and replacing it with a substantially meeker allowance of nutriment. Usually such diets last a few days to a week and are implemented prior to an upcoming event that the dieter wants to lose weight for.

Such practices can be unsafe, especially if done frequently, over a longer period of time, or without the appropriate intake of water and nutrients.
I started a crash diet on wednesday so I can fit into my prom dress this weekend.

Stephanie passed out on the field and had to be taken to a hospital; she had been on a crash diet and was undernourished.
by patty hearst August 06, 2006
 
2.
a good way to gain weight
look at me,im on a crash diet,im losing muscle mass,my metabolism is slow as hell,im smaller but i still have the same body fat,and everything i eat is stored as fat,YAY!!!
by luke19 January 22, 2008
 
3.
A diet plan that basically starves it's followers.
usually consisting of
2 fruit a day
and
a vitamin pill
dumbblonde:i'm doing the new crash diet
smartkid: so what did you eat?
dumbblonde: 2 bananas and a vitamin pill
smartkid: OH MY GOD! YOU RETARD!
by Caitlin215 July 14, 2006