I. noun
Date: 1551
1. : something that happens : occurrence
2. : an event or series of events designed to evoke a spontaneous reaction to sensory, emotional, or spiritual stimuli
3. : something (as an event) that is particularly interesting, entertaining, or important
II. adjective
Date: 1977
1. : very fashionable : in
a happening hairstyle
2. : offering much stimulating activity
a happening dance club