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”