His glasses were misted up, making it difficult to see the path.

misted 是动词 mist 的过去分词形式，mist 的意思是"起雾、蒙上水汽"。misted up 表示"被雾气覆盖"或"起雾模糊"，这里指眼镜镜片因温差产生水雾而变得模糊不清。