Same language but Different

I just saw a post by some one and its made me think and now I have to ask you all , when did the word and how did the word "Fanny" go from being in English a womans front bum (l) to in American English a rear Bum ( _ ! _ ) did it happen as they left the Britain shore line and landed in America or some other way?, asking the sages in SW as I know there will be no Arseing around with the answer as Ass's mean a lot to you all ( I shouldn't laugh at my own jokes but some times I have too lol ) xx