This word is the natural predecessor of 'Chillax'. First coined in Brighton in the late 1990's, 'Chool' is an amalgam of 'Chill out' and 'Cool it'. Where as 'Chillax' is often used to positivley describe ones own activity, or intended activity, 'Chool' has a tone of pejorative instruction.
You're being a bit weird mate, why dont you chool?
If Hannah dosen't chool a bit, her head will explode.