A knock is referring to someone who is a
fiend for
drugs of any kind.
weed, coke, pills, meth etc... it’s like calling someone a druggie, drug-head , tweaker, fiend, or addict.
“He pawned his family’s jewelry and
tv to get
drugs that guy is a straight up knock”
Drug dealer
may say: “I’m going to go serve one of my knocks a gram and I’ll be back in 30”