The saying means what is considered good is in fact bad and what is considered bad is actually good.
an example could be you trust someone and they seem to be a good person but looks are deceiving and they turn out to be bad
Hi Karen,
Sorry I can't come to your birthday party, I REALLY wanted to come. The only thing is I had to walk my cat... no I mean my dog. Yes my dog. Sorry I just got a dog yesterday I have to walk it. The vet said my dog had to be walked for however long your party is hours a day. Sorry about that. Toodles
sincerely, Chad
Which word is in italics?