un ‧ der ‧ tow /ˈʌndətəʊ $ -dərtoʊ/ BrE AmE noun [singular]
1 . 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.
2 . 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.