be sad, be dejected, fear
怛 centers on a heavy, downcast emotional state: sadness, dejection, and the fear that often accompanies deep distress.
怛 combines the heart radical 忄 with 旦, which likely contributes a phonetic element. The exact historical development is uncertain, but the heart component points to an emotional state.
The heart radical 忄 beside 旦 (dawn) suggests a heart weighed down at daybreak—feeling sad, dejected, or afraid as the new day begins.
For ダツ, imagine a sad person muttering "dats just my luck" in dejection: dats -> ダツ, and the heart sinks with fear.