The characters combine to create a vivid image: the “air of joy” (喜气) is “vast and overflowing” (洋洋), describing a scene or a person completely saturated with a festive spirit.
In Chinese culture, joy is often a communal experience. Important events are meant to be celebrated loudly, visibly, and with many people. The concept of 热闹 (rènao), or a lively and bustling atmosphere, is highly valued. 喜气洋洋 is the perfect expression of this cultural value. It captures the ideal state for a celebration: a vibrant, auspicious, and shared happiness that everyone can see and feel. A Western comparison might be the phrase “full of holiday spirit,” often used around Christmas. However, there's a key difference. “Holiday spirit” can be a quiet, internal feeling of warmth and nostalgia. 喜气洋洋, on the other hand, is almost always external and observable. It describes the bright red decorations, the smiling faces of a crowd, the cheerful music, and the overall jubilant energy of an event. It's less about quiet contentment and more about radiant, overflowing festivity.
This idiom is widely used in both spoken and written Chinese, especially in contexts related to celebrations.