'Friday' Star John Witherspoon -- Robin Williams Was A Coke Fiend

John Witherspoon has thrown cold water on the homages to Robin Williams ... saying he was a hardcore junkie.

Witherspoon -- who played the father in "Friday" -- worked with Robin back in 1977 on the set of "The Richard Pryor Show" ... and tells us not only was Robin an addict, but that he even stole his coke right out from under him.