Going down the rabbit hole, I came to this article.
Why is it called a restroom, anyway? - The Straight Dope
SDStaff Dex wrote:
[N]ow seems like a fine opportunity to work our way through the long list of toilet euphemisms and their origins, so far as is known.
What, no love for ‘head’? Growing up in a Navy family in a Navy town, this term was in common usage when I was very much shorter than I am today.