Stories We Like: A Guide to the Comma
Old English swimman "to move in or on the water, float" (class III strong verb; past tense swamm, past participle swummen), from Proto-Germanic *swemjanan (cf. Old Saxon and Old High German swimman, Old Norse svimma, Dutch zwemmen, German schwimmen), from PIE root *swem- "to be in motion."
The root is sometimes said to be restricted to Germanic, but possible cognates are Welsh chwyf "motion," Old Irish do-sennaim "I hunt," Lithuanian sundyti "to chase." For the usual Indo-European word, see natatorium. Sense of "reel or move unsteadily" first recorded 1670s; of the head or brain, from 1702. Figurative phrase sink or swim is attested from mid-15c., often with reference to ordeals of suspected witches.
1540s, "the clear part of any liquid" (above the sediment), from swim (v.). Meaning "part of a river or stream frequented by fish" (and hence fishermen) is from 1828, and is probably the source of the figurative meaning "the current of the latest affairs or events" (1869).
To perform well; succeed; fly: I didn't think the Harptones quite swam last time I saw them
[1970s+; perhaps fr sink or swim]