Exischuo intensifies ischuo (to be strong/able) with ex (fully/out). It appears once — Ephesians 3:18 — in Paul's great prayer that believers may 'have strength [exischusete] to comprehend... what is the breadth and length and height and depth, and to know the love of Christ that surpasses knowledge.' The word implies that the comprehension of divine love requires a capacity beyond ordinary human strength — a divinely empowered ability.
Paul prays for exischuo because the love of Christ requires more than ordinary human cognition. To grasp what cannot be fully grasped — to 'know the unknowable' — requires the Spirit's empowerment. The four dimensions of love (breadth, length, height, depth) are not geometrical; they signal infinity. No finite mind can contain the infinite love of God unaided. Yet Paul prays that they would grasp it — because in the Spirit, the incomprehensible becomes experienced truth. The prayer is the answer: ask for strength to know the love that surpasses knowledge.