something is rotten in (the state of) Denmark

Something strange or suspicious is going on. The line comes from Shakespeare's Hamlet. Why is Katherine being so nice to me all of a sudden? I'm telling you, something is rotten in the state of Denmark. "Something's rotten in Denmark," the detective muttered as he looked through the surveillance photos.

Prov. something suspicious is going on. (From shakespeare's play Hamlet.) Jim: Look, there's a light on in the office, even though it's way past the time everyone should have left. John: Something is rotten in the state of Denmark. Jane: I wonder why Fred is coming in so late every morning. Jane: Something is rotten in Denmark.

Something is seriously amiss; there is a smell of corruption. This expression is a direct quotation from Shakespeare’s Hamlet (4.1). Eric Partridge suggested that rotten was originally an analogy to cheese, for which Denmark has long been famous, and that possibly the expression was a catchphrase even when Shakespeare used it.