in the back of mind
*in the back of someone's mind
Fig. remembered by someone, but not very important; vaguely remembered by someone. (*Typically: have something ~; keep something ~; leave something ~; put something ~; remain ~; Stay ~.) You should put this problem in the back of your mind and concentrate on other things.
in the back of your mindalso at the back of your mind
understood or known but not actively considered I'd like to believe we can still be friends, but in the back of my mind, I know that's not true.