His vague explanation only muddied the situation further.

muddied 是动词 muddy 的过去式，muddy 由名词 mud（泥巴）加上后缀 -y（表示"充满...的"）构成，本义是"泥泞的"，引申为"使混乱、使模糊不清"的意思。这里表示"使情况变得更加混乱不明"。