Ask yer sister!

exp.

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The only effective retort to an insult of one's matriarchal parentage.

In use

Johnny: "Is yer mudder workin' down de docks again tonight boy?" Mikey: "Ask yer sister!"

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

Ahoy there. Nautical, I know. Would you like me to translate 'Ask yer sister!' in use for you?

'Is your mother working down at the docks again this evening, old boy?' 'Do enquire of your sister!'