n. 1. A current below the surface of water, sometimes flowing in a contrary direction to that on the surface. Totten. [ 1913 Webster ]
2. Hence, figuratively, a tendency of feeling, opinion, or the like, in a direction contrary to what is publicly shown; an unseen influence or tendency; as, a strong undercurrent of sentiment in favor of a prisoner. [ 1913 Webster ]
All the while there was a busy undercurrent in her. G. Eliot. [ 1913 Webster ]