Mayonnaise n' Mustard 

Simple as dat. Formally used by bay area rapper "Pill Clinton", a.k.a. Mac Dre. To explain a complex situation with a short connotation, such as "thats that".

We hopped over there, mayonnaise n' mustard, yyyyaaaaaddddaaaammmeeaaannnnn??

Mayonnaise n' Mustard by Jah Light, Bob. December 18, 2008

Bung Mustard 

Firey, bung burning, explosive diarrhea. Like liquid acid pouring from your back door. Leaves the oring stinging for some period of time.

Oh man the bung mustard is really bad. I may have food poisoning. My ass is on fire!!!

Bung Mustard by Eaton Holgoode April 12, 2017

Eating a burger with no honey mustard 

Eating a burger with no honey mustard

“Eating a burger with no honey mustard, Eatin’ a burger with no honey mustard”

Eating a burger with no honey mustard by Zhongli January 4, 2021

Gutter Mustard 

When someone is so drunk they walk out of the bar and puke in the gutter or on the curb. The vomit is normally a yellow hue from all the beer they have consumed; like mustard.

Oh look there goes Jim he is making Gutter Mustard again

Gutter Mustard by Hearevil October 1, 2010

The Hen's Mustard 

Used colloquially to mean an unnecessary addition of something 'Exciting' to what is already considered a functioning - if boring activity/object/product

"I used to quite like Dave's car, but that spoiler really is the Hen's Mustard"

The Hen's Mustard by navarrone906 March 14, 2017

Sharty Mustard Butt 

When somebody sharts in their underwear and wallows around in it for a long period of time. The color and texture resembles a mustard like substance.

Brian is a sharty mustard butt.

Sharty Mustard Butt by KrissyNJbo June 19, 2010

Playing Mustard 

Involves working your ass off for a few days prior to taking time off work, as opposed to playing catch-up (ketchup) upon your return to the office afterwards.

"Hey Bryan, you wanna duck out early today and play a quick 18 with Frank and Toby down at the country club?"
"I wish man. I’ll be in the office playing mustard for the next 3 days before Krista and I head to Bermuda for our 10-year anniversary next week."

Playing Mustard by Cheeky Mirth February 4, 2016