fearful, nervous, timid
忑 centers on an unsettled, anxious state of mind: feeling fearful, nervous, or timid. The meanings all point to a heart that is not at ease.
忑 combines 下 (down, below) with 心 (heart), suggesting a heart that is sinking or low, which fits the fearful and nervous meanings. The exact historical development is uncertain, but the component arrangement is consistent with the idea of a downcast heart.
The heart 心 is under 下, like a heart sinking down with fear. When your heart drops, you feel nervous and timid.
For トク, imagine a timid person who is too nervous to talk, so they just give a small 'tock' sound like a quiet clock: tock -> トク, and the fearful heart stays silent.