Green’s Dictionary of Slang

chill (out) v.

1. [1950s] (US Und.) to get rid of the victim once the confidence game has reached its climax and he has been robbed.

2. [1960s+] (orig. US) to calm down, to control one’s emotions, to relax, to act ‘cool’; often as imper.

3. [1980s+] (orig. US) to pass the time of day, to ‘hang out’; usu. as chillin’ n.

4. [1980s+] (US drugs) by ext. of prev., of a dealer, to work on a street corner or in a bar.

5. [1990s+] to calm someone down.

6. [1990s+] to take a break, to pause.

7. of events, to calm down.