stupid, foolish
騃 centers on a lack of intelligence or sense: being stupid or foolish. The meanings are straightforward and closely related.
騃 combines 馬 (horse) with 矣, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with the idea of stupidity or foolishness.
Picture a horse (馬) standing beside a shape that looks like a person with a big head (矣), acting foolishly. The horse seems to be thinking, 'That human is so stupid!'
For ガイ, imagine a foolish guy trying to ride a horse backwards: guy -> ガイ, and his stupidity is on full display.