Seems that no one knows how this originated so let me break it down. It first appeared on the saturday morning cartoon Batman, where Batman approaches Freeze and Mr. Freeze says im chillin like a villan, hence why he was chillin (literal definition).
Batman: Freeze what are you doing?
Mr. Freeze: chillin like a villan
A tag that originates from the village of Brockenhurst, Hampshire, United Kingdom. It stands for Boys of The Wood, as Brockenhurst is in the New Forest. One tag has been found in Sway, a nearby village. It was found on a fence down a rather mouldey smelling alley-way, on the way to the village from Nick's house.
Nick: Hey Josh, you gonna do some tagging tonight?
Josh: Yeah B.T.W. is gonna be sprayed on every fence within a mile of Brock!
Nick: Yeah well I'm Sway for life so I can't join you tonight, maybe you should move to Sway.