timorous, nervous
忐 centers on a state of inner unease: feeling timorous, nervous, or unsettled. The given meanings describe a fearful, anxious disposition.
忐 is a relatively rare character. It combines 上 (up) and 心 (heart), likely suggesting a heart that is raised or unsettled, conveying nervousness. The exact historical development is uncertain.
Picture a heart (心) that is jumping up (上) into your throat from nervousness. 忐 is that timorous, jittery feeling.
For クン, imagine a timid person named Kun who is always nervous. Kun -> クン, and his heart jumps up whenever he's scared.