和英辞書

しい

suffix

used to form adjectives from nouns, reduplications, or the 未然形 (mizenkei, “incomplete form”) of verbs; an adjective derived by this suffix implies a thorough and certain possession of the quality of the suffixed term:

彼の行動は子供しいと思います。

I think his behavior is childish.

意味（1）

used to form adjectives from nouns, reduplications, or the 未然形 (mizenkei, “incomplete form”) of verbs; an adjective derived by this suffix implies a thorough and certain possession of the quality of the suffixed term:

from nouns: 大人(おとな) (otona, “adult”) + しい (shii) → おとなしい (otonashii, “quiet, gentle”)