adverb · behaving in an obedient or well-behaved manner, typically used for children

他坐在那里乖乖地看书。

noun · a term of endearment used for a child or a loved one; sweetie

乖乖，我们该回家了。

noun · An exclamation expressing surprise or amazement; goodness gracious!, oh my lord!

乖乖！这鱼长得真大。