blow me down!(is a slang expression that was used by sea pirates and seafarers in the 17th and 18th centuries. It is a way of expressing surprise or disbelief Taras)
that'll be the day("He invites us all to a party at his own house" "That'll be the day!" – "Он приглашает нас всех к себе домой" – "Не может быть!" VLZ_58)