confused, stupid, dull, senile
惛 centers on a state of mental cloudiness: being confused, dull, or slow-witted, and by extension senile. The meanings all describe a mind that is not sharp or clear.
惛 combines the heart radical 忄 with 昏 (dusk, dark). The character likely expresses a darkened or obscured mind, leading to confusion or dullness. The exact historical development is uncertain, but the component combination is consistent with the meaning.
The heart radical 忄 beside 昏 (dusk) suggests a mind that has grown dim like twilight. Picture your thoughts becoming as murky as dusk, leaving you confused and dull.
For コン, imagine a confused person trying to con someone but failing because their mind is too dull: con -> コン, and the dim-witted scheme falls apart.