It's a very old-fashioned phrase isn't it? Something I always see in 19th century novels when they're trying to imply that someone (always a woman I thought!) has been flirting with boys or maybe even lost their virginity before marriage! I take it to mean that not only are they no better than they should be, they actually aren't as "good" as they should be - but it's a moral judgment isn't it rather than one relating to class? Or have I got it wrong?There's that bizarre phrase, "S/he's no better than s/he ought to be", I mean what the fuck does that actually mean?
Edit: google says that it does relate to (sexual) morality but may refer to class in that it suggests someone's sexual morals are limited by their class.