Green’s Dictionary of Slang

the act of relaxing, hanging out.

1990		N.Y. Mag. 28 May 27/1: [black high school slang] ‘Chilling hard’ (hanging out).
2000		Observer Rev. 21 May 5: I’m thinking of going back to school. Hanging out, chilling.
2001		(con. 1990s) in J. Miller One of the Guys 56: ‘[It] be kinda fun, you know, bein’ around all your little friends, just chillin’’.