distress, difficulty
阸 centers on being in a tight, blocked, or difficult situation: distress, hardship, or a narrow, obstructed place.
阸 combines ⻖ (mound, hill) with 戹, which likely contributes the sound and the idea of narrowness or obstruction. The exact historical development is uncertain.
The left mound ⻖ represents a steep hill, and the right side 戹 looks like a person trapped in a narrow space. Together, they show being stuck in a difficult, narrow pass.
For ヤク, imagine a yak stuck in a narrow mountain pass, distressed and unable to move: yak -> ヤク.