Jenny a friend in Surrey has recently had an operation for a bladder problem. I haven't seen her for a long while and rang to check how she is after surgery. It's not a procedure I've heard of before and had to look up details on the Internet. Later when talking with a mutual friend we were wondering just how painful the recovery period will be. I was trying to say that sitting on her fanny could be pretty tricky for a while. But I found myself self censoring my words as I was talking with an older more ladylike friend.
Afterwards I had a vague memory that Fannies were women in one of the armed services. So I tried to do a google search. Eventually I found what I was looking for but not without many strange unrelated results.