My back got lonesome
What you tell your main squeeze to explain why you invited a second hot chick/hunk to come over and spoon you from behind in bed, while you are snuggling your Number One partner in front.
Telling your significant other dat "My back got lonesome" may indeed alleviate his/her emotional discomfort --- i.e., confusion and/or concerns about your wanting to have a "threesome snooze" --- but you should still remember to perform da wrist-twist compromise wif him/her. so as to maintain said forward-located slumberer's physical comfort, as well.