“Tweener” is officially defined as “a person or thing considered to be in between two other recognized categories or types.”
This is a spot-on explanation of how the term is used in football.
Tweeners are players without a clear and obvious position, typically because their frame leaves them in no man’s land between the size thresholds of two different positions. They are just small enough to conceivably line up at one position, where they would be bigger, but still big enough to feasibly line up at another position, where they would be on the smaller size.
Take John Franklin-Myers, for example.