The floor in the room became much dustier after being left uncleaned for so long.

dustier 是形容词 dusty 的比较级形式。dusty 由名词 dust（灰尘）加上后缀 -y（表示"充满...的"）构成，意思是"布满灰尘的"。dustier 表示"更加布满灰尘的"。