I haven't heard the "back of" expression you speak of, but I'll give you an equivalent --
I have heard many Americans say "back side" instead of "butt".
I grew up with "butt" being a sufficiently polite term for that part of the body.
I mean, there's the butt of a rifle, and the word is basically equivalent to "rump" is it not?
But since it still refers to the same object, that body part so close to where excretion takes place, we have to hop back on that old euphemism treadmill...
Another great example of the Euphemism Treadmill is the word to refer to people with brown skin.
In the past, they were referred to as negroes (from "negro", the Latin word for black). Even the dreaded slur "nigger" came from this -- just a verbal corruption of the word "negro". It's easy to see where that slang came from. But at some point -- again, is being black inherently an insult? -- it took on a negative connotation. It's like they were referred to as what they were with a hate-filled, snide tone, and the slur was born?
I don't know what time period exactly, but next they were called "colored".
In the 80's in America, we called them blacks. The term was descriptive, scientific; not derogatory at all. (And those who used the term, including the "Whites", were fine being called Whites even though almost none of them were albino) But apparently that wasn't good enough. Apparently it's an insult (or becomes an insult) to be of that race? Because back on the treadmill we go... afro-american, african american, etc.
I consider this the "euphemism treadmill" because all these terms were not inherently insulting. I have certainly encountered some insulting terms, which never truly applied to this or any other race. But why shy away from a useful, descriptive term which "worked"?