内緒での意味でbehind someone's backといい、go behind someone's backは裏でこそこそする、talk behind someone's backは陰口を言うくらいの意味です。0:13
http://www.youtube.com/watch?v=qjLVNJ0ieMQ 2:02 you don't know people are talking behind your back or laughing at you.
http://www.youtube.com/watch?v=nj0j04C_a_o 1:02 to her face he was the doting suitor but behind her back he was pure evil. 面と向かってはメロメロの求婚者、裏では悪党
