sincere
忱 centers on heartfelt sincerity: a deep, genuine feeling or truthfulness from within.
忱 combines the heart radical 忄 with 冘, which likely contributes the sound. The heart component points to sincerity as a feeling rooted in the heart.
The heart radical 忄 beside 冘 suggests a sincere feeling: imagine a heart that is open and true, with 冘 as a person bowing deeply in genuine respect.
For シン, picture a sincere person bowing so deeply their shin touches the ground: shin -> シン, and the heart shows true feeling.