To fail a drug test or piss test. Drug tests are usually given by either a probation officer or a parent/guardian. The test requires for you to urinate in some sort of container, while afterwards your urine is tested for illegal substances. If you test positive for any illegal substances, you just dropped dirty.
Dude, my PO is comin' tomorrow. I'm gonna drop dirty for sure.
by Miss Kristin July 17, 2006
Dude! That mexican food gave me a case of the drop dirties.
by charlie muphy August 23, 2009