Green’s Dictionary of Slang

1. carrying a weapon.

1947

I. Shulman Amboy Dukes 114: All you boys were hipped except you and Frank.

2. carrying a hip-flask.

1920

Variety 31 Dec. 8: It was a great idea, but where were they gonna get it unless the college boy was hipped? He sure was — the real old stuff [HDAS].