Much like "jet wash" (the rapidly moving gases expelled from a jet engine), "Belch Wash" is the preferably vile and unpleasant gasses produced by a belch expelled in the proximity of another person who unknowingly and unavoidably walks through it, ingesting the odorous contents of the belcher's stomach and expresses a negative reaction.
After a couple Scotch on the rocks at the bar, I enjoyed a fine halibut filet in garlic cream sauce, steamed broccoli with jasmine rice. Rich and delicious! While walking out of the restaurant I quietly expelled a hot gaseous belch. My unsuspecting wife walking behind me passed right into my Belch Wash, almost yarped and exclaimed “Thanks, I just walked through your Belch Wash"