means A small body of still or slow-moving fresh water, smaller than a lake and often standing in one place.

from From Middle English 'ponde,' which is really just a variant of 'pound' — yes, the same word as the enclosure where stray animals were impounded. Both come from Old English 'pyndan,' to dam or shut in. So a pond is, etymologically, water that's been penned up. Whether the watery 'pond' or the dog-catcher's 'pound' came first is genuinely murky, but they're the same root holding things in place.