shame
忸 centers on the feeling of shame, a painful self-consciousness tied to one's actions or state.
忸 combines the heart radical 忄 with 丑, which likely contributes the sound. The exact historical development is uncertain, but the heart component suggests an emotional state like shame.
The heart radical 忄 beside 丑 suggests a heart weighed down by an ugly, shameful feeling. Picture your heart sinking with shame.
For ジク, imagine feeling shame because you accidentally said a rude 'joke' (ジク) in public. The joke -> ジク hook ties the sound to the heart's shame.