鬼の空念仏
((O)ni no sora-nenbutsu; “An oni‘s empty prayers”)

Definition:

A cruel and cold-hearted person putting on a show of benevolence, compassion, or charity. A bad person behaving uncharacteristically well. Like a demon making making a big show out of praying, but only in order to get out of trouble or otherwise benefit in some way.

Breakdown:

This is a simple noun phrase. We start with 鬼 (oni) a Japanese “demon” or “ogre,” generally used in kotowaza as an exemplar of evil. The associative particle の (no) links the oni to a noun comprising 空 (sora), “empty,” and 念仏 (nenbutsu), “Buddhist prayer.”

Notes:

Two of the を entries of the iroha karuta sets begin with 鬼. This phrase is not one of them, but it struck me as useful, so here it is.

This phrase may be shortened to 鬼の念仏, but if 空 is used, make sure to pronounce it sora – in some contexts kara would be an appropriate reading, but here that would be considered an error. Compare a variety of expressions including 空言 (soragoto), “lie,” 空泣き (soranaki), “crocodile tears,” 空寝 (sorane) “pretending to be asleep,” and my favorite, 空耳 (soramimi), “mishearing,” or specifically misinterpreting song lyrics for humor.

Example sentence:

よく先生の前で苛めっ子達が苛められたクラスメイトを褒めて見せるが、鬼の空念仏に過ぎない自分のための言葉だ。

(“Yoku sensei no mae de ijimekko-tachi ga ijimerareta kurasumeito wo homete miseru ga, oni no soranenbutsu ni suginai jibun no tame no kotoba da.”)

[“In front of the teacher, the bullies often make a show of praising the classmates they bully, but it’s nothing more than self-serving hollow piety.”]

Also this blatantly insincere self-aggrandizing BS.