quiet
頠 centers on a state of quiet composure or gentle stillness. The given meaning 'quiet' points to a calm, dignified silence rather than just absence of noise.
頠 combines 頁 (head) with 危, which may contribute a phonetic element. The exact historical development is uncertain, but the character is associated with a quiet or composed demeanor.
The left side 危 suggests danger or precariousness, and the right side 頁 is a head. Imagine a person in a dangerous situation who remains perfectly still and quiet, keeping their head calm.
For ガイ, picture a quiet guy meditating with a calm head: guy -> ガイ, and his stillness matches the quiet meaning of 頠.