1
general::
Phrase(s): sink or swim
Fig. to fail or succeed. (Alludes to the choices available to someone who has fallen into the water.) • After I’ve studied and learned all I can, I have to take the test and sink or swim. • It’s too late to help John now. It’s sink or swim for him.
McGrawhill's American Idioms And Phrasal Verbs
2
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Succumb or succeed, no matter what, as in Now that we've bought the farm, we'll have to make a go of it, sink or swim. This expression alludes to the former barbaric practice of throwing a suspected witch into deep water, often weighted down. In case of sinking, the victim died; in case of swimming, the victim was considered in league with the devil and therefore was executed. A related idiom, float or sink, was used by Chaucer in the late 1300s; Shakespeare had the current form in 1 Henry IV (1:3): "Or sink or swim."
American Heritage Idioms