Obedient
Quiet, Calm
い adjective
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!
大人しい犬
an obedient dog
大人しい人
a quiet person
大人しい子
a quiet child
大人しい犬ですね。
That's a quiet dog.
女王は小さいころ、とても大人しかった。
The queen was very quiet when she was little.
今日はホテルで大人しくするよ。
I'll be relaxing at the hotel today.