From Longman Dictionary of Contemporary Englishbe the last thing on somebody’s mindbe the last thing on somebody’s mind (also be the furthest thing from somebody’s mind)THINK ABOUT# to be the thing that someone is least likely to be thinking about Insurance was the last thing on my mind when we set off that day. → mind
Examples from the Corpus
be the last thing on somebody’s mind• Marriage is the last thing on my mind right now. 