An abbrieviation of "Free The Weed". Used commonly in graffiti in countries where the use of cannabis for recreational purposes is prohibited and is refering to the legalisation of cannabis.
Oh poor souls who think FTW means For The Win! jami93, a Finnish boy (*cough*a troll*cough*) who often comments to a webcomic Sosiaalisesti Rajoittuneet has told us the REAL meaning of FTW.