Is it the word "between", but truncated, or a portmanteau of "in between" and "teen"?
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Here's Merriam-Webster's definition of portmanteau:
Here's The Free Dictionary's definition of tween:
The OED says tween is short for tween-age, after teen. For tween-age, they say it's an alteration of teenage after 'tween (a form of between).
So, yes, tween can be considered a portmanteau of "[in] between" and "teen".