Mustard Box Share definition Flag A 5-15 year old car that isn't quite shitty enough to be called a shitbox, and isn't economical enough to be a ecobox. "Greg bought himself a mustard box, he fell in love with that thing. It's a 2004 VW Passat" Mustard Box by Arionio March 6, 2013 0 0 Get merch Get the Mustard Box mug. Buy Now — $32.95 See more merch