He was overwhelmed with happiness when he saw he won first prize.

overwhelmed 是动词 overwhelm 的过去分词形式，表示"被...淹没、被...压倒"。over- 是前缀表示"过度"，whelm 来自古英语表示"淹没"。这里 overwhelmed with happiness 形容快乐的情绪如此强烈，就像把人淹没了一样，中文常译为"喜不自胜"或"欣喜若狂"。