dull, dull-minded, simple, stupid
獃 centers on a lack of mental sharpness: being dull, simple, or stupid. The given meanings all point to a slow or dim-witted mind.
獃 is a phono-semantic compound: 犬 (dog) suggests a dull or foolish animal, and 豈 likely contributes the sound ガイ. The exact historical development is uncertain.
A dog (犬) beside a decorative stand (豈) looks simple and unthinking. Picture a dull-minded dog staring blankly at the stand, too stupid to move.
For ガイ, imagine a dull guy staring at the dog: guy -> ガイ, and his blank expression matches the kanji's meaning of stupidity.