When a particularly sick burn is wrought, a hype-man (or woman) may exclaim "That's straight up rancid" to indicate their assent as to the magnitude of the damage. Etymology derives from the process of meat putrefaction. Word typically employed by dashing young rogues
Bob (to impending burn victim): Dude you look like the kid from Home Alone grew up and became a hobbit
Neutral third party (awed): Damn man - that's straight up RANCID
A rancid reminder is the act of taking a shit earlier in the day and not flushing it and leaving it there. Once you return to the toilet later on and take a piss on the old shit the smell that is let off is known as a rancid reminder.