trailing sickness, sick radical (no. 104)
疒 centers on the concept of sickness, especially a lingering or trailing illness. It is primarily used as a radical component in other characters related to disease or ailment.
疒 is a pictograph of a person leaning on a bed or support, representing sickness or infirmity. The modern form retains the leaning shape with two small marks indicating ailment.
The shape looks like a person leaning against a wall with two small drops of sweat or pain marks: a trailing sickness that won't go away.
For ダク, imagine a sick person saying 'daku' (like 'duck') weakly, as if their voice is trailing off from illness: daku -> ダク.