Water lying below the surface of the ground, the ocean, etc.; (occasionally more generally) water at a lower level. Also in plural, esp. in later…

figurative. An activity, force, tendency, etc., of a suppressed or underlying character.

A sea-current below the surface of the water, moving in a contrary direction to that of the surface current.

An underlying or hidden motion or action.

A swell below the surface; an undercurrent.

= nigger in the woodpile n. at phrases 2b.

a concealed motive or unknown factor affecting a situation in an adverse way.

An undercurrent. Also figurative.

In phrases of the type —— in the woodpile: a situation which is adversely affected by a concealed motive or unknown factor.

A subdued note; an undertone or suggestion.

figurative. An underlying tone (of feeling, etc.); a subordinate or unobtrusive element; an undercurrent.

An undercurrent; figurative a tendency beneath the surface of things.

figurative. The underside or inferior part of something, which is often unnoticed or concealed.