放开肚皮
My understanding that 放开肚皮 means to loosen one's belt in the context of indulging in gluttony. Do you literally have to wear a belt and to loosen it up or is it just a manner of speaking?

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Means just help yourself!
we would like to say 放开肚皮，好好儿吃，多吃点儿！

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Is commonly said 放开肚皮 “吃 , which means people familiar with each other or friends together to enjoy food and drink

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It is a manner of speaking which means  'my friend, feel free to eat as much as you can!'
Help yourself

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please eat heartily , fill your stomach with delicious food, have a good appetite!

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Hey, guy, get ready to pig out at something. Don't be shy.