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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.
 
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