SFO to ORD a few months back on a 753, boarding nearly completed. I'm seated in front of E Cabin, near the exits and Lav between First and Econ. People behind me think they will have an empty middle seat then right before the door closes some guy gets on wearing a puffer jacket (its quite warm outside) and smells like he just smoked the entire inventory of a dispensary. It was an unbelievable stench. 4+ hours followed of this guy getting up and out that middle seat over and over and leaving a skunk trail of stench to that Y/J Lav. Everyone was utterly disgusted in the area. Some children in the area breathing it too. Crew did nothing.