mistake, fault, offence
愆 centers on a moral or procedural misstep: a mistake, fault, or offence that deviates from what is correct.
愆 combines 衍 (overflow, excess) with 心 (heart/mind), suggesting a heart that oversteps or goes astray, leading to fault or mistake. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The top 衍 looks like water overflowing its path, and the bottom 心 is the heart. Imagine your heart overflowing with impulse, causing you to make a mistake or offence.
For ケン, picture a stern judge named Ken pointing out your fault: Ken -> ケン, and his judgment highlights the mistake.