Start on about
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
Used in informal settings among friends or family, not in professional or formal situations. 
Whenever we have a family dinner, my uncle starts on about his fishing adventures.
Pro Tip 2/3Potentially Annoying
Implies the speaker may be talking too much or in a way that could irritate the listener. 
At the party, she started on about her diet, and everyone looked bored.
Pro Tip 3/3Subject Focused
Often introduces a specific subject that the speaker has a habit of discussing repeatedly. 
Mike started on about his car repairs again, despite our disinterest.