カ, コク, べ.き, べ.し, よし
can, passable, mustn't, should not, do not
JLPT0 G5 S5 F314
The relevant oracle bone form of this character is a representation of a bent hook + 口 mouth → bend/cup a hand about the mouth to amplify the voice in shouting → shout/laugh/sing in a loud voice. *Possible* is an extended meaning (← make it possible to be heard at a distance), with *approve* by further extension (← approve of what has been made possible).
[Full Kanji Etymology List]