This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The historical origin of 忈 is uncertain. It visually combines 二 (two) over 心 (heart), but the intended meaning and development are unclear.
With two 二 over a heart 心, imagine two hearts beating together, but the exact meaning is unclear.
For ジン, imagine two hearts 心 beating in a gym: gym -> ジン, but the meaning remains uncertain.