a tendency underlying or at variance with the obvious or superficial significance of words, actions, etc.: Even in his friendliest remarks, one could sense an undercurrent of hostility.
2.
a current, as of air or water, that flows below the upper currents or surface.
1663, "stream of water or air flowing beneath the surface or beneath another current," a hybrid formed from under + current. The fig. sense of "suppressed or underlying character" is attested from 1817.