weep, grieve
慷 centers on deep emotional sorrow: weeping and grieving from the heart.
慷 combines the heart radical 忄 with 康, which likely contributes the sound コウ. The exact historical development of the compound is uncertain.
The left side is the heart 忄, and the right side 康 looks like a person bent over in sorrow. Picture a grieving heart that makes you weep.
For コウ, imagine a grieving person letting out a deep, sorrowful 'coo' sound: coo -> コウ, as they weep from the heart.