Answer:
In this sentence, the word "septic" most closely means "disgusting."
Step-by-step explanation:
The word "septic" is used to describe something that is infected with harmful bacteria, causing decay or putrefaction. In this context, the narrator is saying that the atmosphere, which he had found amusing before, had now turned unpleasant, contaminated, and perhaps even toxic. So, the word "septic" is being used metaphorically to suggest that something that was once enjoyable or amusing had turned rotten or unpleasant. Therefore, "disgusting" is the closest in meaning to "septic" in this context.