The filthiness of the kitchen was disgusting, with grease and food scraps all over the floor.

filthiness 是由形容词 filthy（肮脏的）加上名词后缀 -ness 构成的名词，表示"肮脏的状态或程度"。filthy 本身来自名词 filth（污物、肮脏）加上形容词后缀 -y。