(This phrasal verb has more than one meaning and can't be separated): to repeatedly talk about the past, sometimes in a way that annoys people. [Quem não conhece alguém assim?]
“He's always harking back to his childhood and saying how things were better then.”
“Don't mention text-messages! She'll hark back to her school days and tell you all about the importance of good handwriting and spelling!”
Source: Knights English