go against, disobey
愎 centers on stubborn resistance: going against what is expected or outright disobeying. The meanings share an idea of willful opposition.
愎 combines 忄 (heart/mind) with 复, which likely contributes the sound. The exact historical development is uncertain, but the heart component suggests a mental or emotional disposition, fitting the idea of stubbornness.
The heart radical 忄 beside 复 (which looks like a doubled-up shape) suggests a heart that doubles down on its own way, refusing to follow orders.
For フク, imagine a stubborn person who refuses to budge, like a fugu (pufferfish) that puffs up and won't move: fugu -> フク, and the heart 忄 stays defiant.
obstinacy