The opposite of clean and has many connotations to go with it. Muddy, messy, ultra-kinked out, super sexy...well, you get the idea.
What did Seifer do to get that tattered and dirty beloved trench coat of his clean after the war? Is Zell one to make a dirty mess in the kitchen when cooking, or doing anything that involves household chores? Is Selphie scared to shag on Irvine's supposedly "dirty" sheets? Does Quistis use special oils to keep her whip from becoming dirty and cracked? Perhaps Fuu is really the messy one in the relationship.
However you paint that dirty picture, give it up in 100 words.