godan verb (-ru)
act like a goody-goody; pretend to be nice
Used when someone puts on a show of being well-behaved or virtuous, often with a negative or mocking tone.
先生の前でいい子ぶるのはやめなよ。
Stop acting like a goody-goody in front of the teacher.
彼はいつもいい子ぶって、本当は何を考
He always pretends to be nice, so I never know what he's really thinking.