only, merely, but
衹 centers on limitation or restriction: only, merely, or but. It marks something as being no more than what is stated.
衹 combines 衤 (clothing) with 氏, which likely contributes the sound. The exact historical development is uncertain, but the character is used to express limitation.
The clothing radical 衤 beside 氏 suggests a garment that is merely a single piece of cloth, nothing more. Think of it as 'only' a simple robe.
For キ, imagine a key that opens only one lock: key -> キ, and the kanji means 'only'.