box one's ears, slap
摑 centers on striking the side of the head with an open hand: boxing someone's ears or slapping.
摑 combines 扌 (hand) with 國, which likely contributes the sound カク and suggests a forceful hand action. The exact historical development is uncertain.
A hand 扌 striking the side of a 國 (country) shaped head: imagine boxing someone's ears so hard their head rings like a gong.
For カク, picture a loud 'crack!' sound as the hand slaps the ear: crack -> カク, and the sting of the slap echoes.