You might as well be bobbin yer ass against de Bridge o' Ross

exp.

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Performing a fruitless activity.

Let Lawrence Muck translate it to posh English for you
Lawrence Muck

What a frightfully fine day to meet you. Would you like me to translate 'You might as well be bobbin yer ass against de Bridge o' Ross' in use for you?

You might as well be bumping your donkey against the Bridge of Ross, for all the good it will do you.