no better than you should (or ought to) be regarded as sexually promiscuous or of doubtful moral character.

☞ This phrase dates back to the early 17th century. Used typically of a woman, it is now rather dated.

1998Spectator 'She's no better than she ought to be.' (British mothers of my generation… often used that enigmatic phrase. They would use it about female neighbours of whom they disapproved, or women in low-cut dresses on television.)