しい

Etymology 1
→ →  → * → * → * →

Developed from, the of adjectival suffix.

Suffix

 * 1) ; an adjective derived by this suffix implies a thorough and certain possession of the quality of the suffixed term:
 * from nouns: +  →
 * from verbs: +  →
 * from Japanese original reduplications: +  +  →
 * from reduplications of Chinese origins: +  +  →