Dirty bomb a conventional bomb that contains radioactive material.
dirty old man an older man who is sexually interested in younger women or girls.
dirty water will quench fire a man's lust can be satisfied by any woman, however ugly or immoral; saying recorded from the mid 16th century.
dirty weekend a weekend spent away, especially in secret, with a lover.
dirty work at the crossroads dishonourable, illicit, or underhand behaviour. The term (recorded from the early 20th century) may reflect a view of crossroads as a sinister place, where suicides were traditionally buried.