The result of a message board or forum thread subject getting overtaken by a joke until the joke is the point of the thread and everything turns inane. Example: The thread was supposed to be a discussion about cars and pollution, but before long a subjest took hold and now it's full of lightbulb jokes.
SUBJEST
Meaning of SUBJEST in English
Slang English vocab. Английский сленговый словарь. 2012