IPA: //ʌnˈteɪ.kən//
KK: /ʌnˈteɪkən/
Not chosen or occupied; available for use or selection.
The seat next to me was untaken during the entire flight.
Untaken → It is formed from "un-" (meaning not) and "taken" (the past participle of "take", which comes from Old English "tacan", meaning to seize or grasp). The word "untaken" means not seized or not grasped.
Think of 'not' ('un-') having something 'seized' ('taken') — that's why untaken means something that hasn't been taken.