cup made of horn, obstinate
觥 centers on a horn-made drinking vessel, and by extension, the stubbornness or obstinacy associated with heavy drinking or the hard material of the cup.
觥 combines 角 (horn) with 光, which likely contributes a phonetic element. The character originally referred to a drinking vessel made from horn, and the meaning 'obstinate' may derive from the hardness of horn or the stubbornness associated with drinking.
The left side 角 is a horn, and the right side 光 suggests light or shining. Imagine a shiny horn cup used for drinking, and the drinker becoming obstinate after too many cups.
For コウ, picture a stubborn person holding a horn cup and saying 'I won't go!' — the 'go' sound in 'won't go' cues コウ, and the obstinacy ties to the meaning.