means To move through water by propelling yourself with your limbs, body, or fins, without sinking.

from From Old English 'swimman,' from a Proto-Germanic root that's a cousin of German 'schwimmen' and Old Norse 'svimma.' Some trace it further back to a Proto-Indo-European root suggesting 'to move, be in motion' — which is fitting, since swimming is really just moving where moving doesn't come naturally.