Verb. A side effect of extreme over indulgence in crystal meth or crack cocaine. Tweekers typically perform OCD-like activities centered around finding small quantities of drugs that were inadvertently "misplaced". Tweekers may be seen on all fours, searching in gutters, obsessively looking under various objects and so on. In certain circumstances, extreme tweeking resembles autism or catatonia.
A character from the TV show "South Park". Tweek is the ADD kid, who is constantly twitching. He often rants and raves about things that only he sees, and is addicted to coffee. Tweek is in love with Craig, and they sex each other.