Like its counterparts in Hebrew and Greek, the Latin word spiritus originally meant breath (as in expire, respiration, etc.) and breath is what you have when you’re alive and don’t have when you’re dead. Thus spirit = breath = life, the aliveness and power of your life, and to speak of a man’s spirit (or soul) is to speak of the power of life that is in him. When the spirit of a man is unusually strong, the life in him unusually alive, he can breathe it out into other lives, become literally in-spiring.
God also has a spirit—is Spirit, says the Apostle John (4:24). Thus God is the power of the power of life itself, has breathed and continues to breathe himself into his creation. In-spires it. The spirit of God, Holy Spirit, Holy Ghost, is highly contagious. When Peter and his friends were caught up in it at Jerusalem on Pentecost, everybody thought they were drunk even though the sun wasn’t yet over the yardarm (Acts 2). They were (“Spirit” in Wishful Thinking, Frederick Buechner).