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1.
The brownish-yellow discharge found in one's undergarment after an evening of junk food and/or bad beer. It can be delivered by flatulence, or a steady leak.
Boy howdy! your drawers are loaded with ass mustard!!!
by biggdogg September 04, 2003
 
2.
1. Produced after anal sex, the substance which seeps from the anus (mostly semen and fecal matter combined) which is commonly used as a condiment.

2. The substance produced when a person used any condiment as an anal lubricant.
After a long night of fucking, Lauren wiped the assmustard off the bed sheets.