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7.
British slang for "Down Syndrome", also slang for "autism" in other areas of the UK.

This slang term was developed so that people could speak about it in public without offending those afflicted with either illness.
It is popular in the "Wales" region.
i'd hate to be afflicted with plex, that guy with plex can't even come up with anything on his own
by rus_IMN November 01, 2012
 
15.
a grudge or problem that results in an argument or dispute short for complex
The 50 cent dis track against Shyne, Fat Joe and others
by SexyOne March 12, 2005
 
16.
to interfear with.to cause physical or mental duress or harm.
"He plexed him"."John is a plex robot"
by kungfublood July 18, 2003