not likely to happenI think her coming back is a pretty unlikely prospect.
unlikely to do something: He’s unlikely ever to find a job again.
unlikely (that): It seems unlikely that she will make the same mistake again.
most/highly/extremely unlikely: It’s highly unlikely that we’ll be invited.
in the unlikely event (=if this unlikely thing actually happens): In the unlikely event that you are not completely satisfied with your vacation, we will offer a full refund.