飘飘欲仙 is a four-character idiom (chengyu) that literally means 'floating, floating, about to become an immortal.' It describes an overwhelming feeling of lightness, joy, or euphoria — as if you are drifting on air, free from earthly cares. You can use this idiom after a great success, a relaxing experience, or even light-headedness from alcohol. A less common, slightly negative sense of 'complacency' (puffed up with pride) also exists, but the positive, dreamy euphoria is the core meaning.