please exlain :" smile, laugh, grin"

[Saudavabimbim]

Please explain for me the reason why we don't use "laugh" or "smile", so it must use " grin" in this sentence.but " smile, laugh, grin" have same meaning is moving mouth .
There is really nothing much you can do to stop it and I'm afraid you'll just have to grin and bear it.

[Beeesneees]

Hi Saudavabimbim,

It's because 'grin and bear it' is a fixed expression. It's an idiom which means to accept an unpleasant or difficult situation because there is nothing you can do to improve it. Although smile or laugh could be used in place of grin, the idiom is unchangeable.
http://idioms.thefreedictionary.com/grin+and+bear+it
http://idioms.yourdictionary.com/grin-and-bear-it
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Cheers m' dears!