fear, be afraid, shy, timid
怵 centers on a fearful, shrinking-back emotional state: being afraid, shy, or timid in the face of something daunting.
怵 combines the heart radical 忄 with 术, which likely contributes a phonetic element. The exact historical development is uncertain, but the heart component points to an emotional state like fear or timidity.
The heart 忄 beside 术 suggests a heart that shrinks back like a plant bending away from a threat. Picture a timid heart flinching at the slightest touch.
For チュツ, imagine a shy person who chews their lip nervously when afraid: chew-ts -> チュツ, and the heart trembles with fear.