Behind my Behind

When someone says so-and-so spoke about you behind your back, it means they were talking (usually bad things) about you without your knowledge. Technically translated, however, it means the person spoke facing your front. So what should be the right phrase then?

"The person spoke behind you." - You'd have heard wouldn't you? Unless its:

"The person spoke wayyyyyy behind you. Dude!"

"The person spoke facing your behind."

"The person spoke behind your behind."

And if you're a cat, speaking behind your back would mean speaking underneath you.