rare, phenomenal, dilute (acid)
稀 centers on scarcity and thinness: something rare, exceptional, or diluted. The phenomenal sense is a natural extension of rarity, while dilute (acid) applies the idea of thinness to a liquid.
稀 combines 禾 (grain) with 希, which likely contributes the sound and the idea of thinness or scarcity. The character originally referred to sparse grain plants, extending to rarity and dilution.
Grain 禾 that is sparse 希 is rare. Picture a field where only a few stalks of grain grow far apart—a rare, thin crop.
For キ, imagine a rare key that unlocks a hidden treasure: key -> キ, and the key is as rare as the grain in a sparse field.