"Tweaker" is a slang term for a methamphetamine addict, and it comes from the term "tweak," which is another name for methamphetamine.

What does it mean to be called a tweaker?

2 slang : a person who illicitly uses methamphetamine and especially crystal meth When he got like this, his neighbors would scream at him and threaten to call the cops, but they were tweakers too.—

What makes someone a tweaker?

Tweaker, or alternate spelling tweeker, an individual addicted to methamphetamine.