/'ʌndətəʊ; `ʌndɚ,to/
n [sing] current below the surface of the sea, moving in the opposite direction to the surface current, esp the current caused by the backward flow of a wave breaking on a beach （海面下与海面水流呈反向流动的）回流（尤指浪拍岸退回形成的）
*caught in an undertow 碰上回流
* The pull of the undertow can drag swimmers out to sea. 近岸回流的冲力能把游泳的人涌到海里.

n [singular]
the water current under the surface of the sea, that pulls away from the land when a wave comes onto the shore
　The dangerous undertow means that swimming is not allowed.
a tendency or feeling that seems weak, but in fact has a strong effect
undertow of
　The humour of the novel cannot hide an undertow of sadness.