intransitive. Of a fish (or other aquatic animal): to swim rapidly, esp. in a short rush (towards bait, after being hooked, etc.).

To be suspended in a liquid with freedom to move; also, to move freely beneath the surface. †Of a fish: to swim.

transferred. Of a fish: To propel itself. Of the tail: To act as a propeller.

intransitive. To swim, as a fish; hence used of underwater swimmers. Also transitive, to fin it, to fin a (or one's) passage, way.