complain of
愬 centers on expressing dissatisfaction or grievance: complaining of something, often with a sense of emotional distress.
愬 combines 朔 (a component associated with the sound ソ/サク) and 心 (heart/mind), suggesting a heart-felt complaint or grievance. The exact historical development is uncertain.
The top part 朔 looks like a person turning away in frustration, and the bottom 心 is the heart. Together, they show a heart full of complaints.
For ソ, imagine someone complaining with a soft, sighing 'so...' sound: so -> ソ, as they voice their grievance.