means The current of water beneath the surface that flows back out to sea after a wave breaks, often felt as a strong pull at the legs.

from A transparent English compound, no mystery to decode: "under" plus "tow," that old word for pulling or dragging (a cousin of "tug" and "tie," rooted in Germanic words for hauling). So literally "the pull beneath" — the water tugging you seaward while the waves above keep rolling in. A 19th-century coinage as a maritime and bathing term, when sea-bathing grew popular and the hidden danger needed a name.