Docile, Quiet
い adjective
おとなしい
さとうさんの犬は、とても大人しいです。
Satou’s dog is very obedient.
なんなのアイツ。がっこうでは大人しいいんキャのくせに。
What's with that guy? He is just a quiet nerd at school.
大人しい子どもだったよ。
He’s an obedient kid.
Remember how you learned that 大人 means "adult?" Well this is the adjective version of that. The I-adjective (see how it ends in い?). What adjective do adults want to be able to use for their own kids? They want to say they are obedient, quiet kids!
You already learned this reading when you learned 大人! You should be able to read this too!