This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
恈 combines the heart radical 忄 with 牟, which may contribute a phonetic element. The exact historical development is uncertain.
The heart radical 忄 suggests an emotion, and 牟 can be associated with greed or seeking. Picture a heart that craves something intensely, but the exact meaning is unclear.
For ボウ, imagine a bow (as in bow and arrow) being pulled back by a greedy heart: bow -> ボウ, and the heart 忄 desires the prize.