It’s basically just a person that absolutely stanks due to a lack of personal hygiene. If you go more than 5 days consecutively without a shower you’re probably a reek-monger.
Steven: “ ugh, did you smell Matt in the hallway? That man needs to take a damn shower for once in his life?”
Marcus: “ Yea facts, I was boutta yack. He’s such a reek-monger.