Slang: NTC's Dictionary of American Slang

Ø  Bark up the wrong tree: to make the wrong choice; to ask the wrong person.

eg. If you think I'm the guilty person, you're barking up the wrong tree.

Ø  Beat a dead horse: to continue fighting a battle that has been won.

eg. Stop arguing! You have won your point. You are just beating a dead horse.

Ø  Beauty is only skin-deep: looks are only superficial.

eg. – Isn't Jane lovely? – Yes, but beauty is only skin-deep.

Ø  Before you can say Jack Robinson: almost immediately.

eg. And before you could say Jack Robinson, the bird flew away.

Ø  Behind the times: old-fashioned.

eg. Sarah is a bit behind the times. Her clothes are quite old-fashioned.

( from mis.bitarafan )