(third-person singular simple present harks back, present participle harking back, simple past and past participle harked back)
The forms harken back and hearken back are also found, and have been used since the 1930s, and the bare form harken has been used since the 1980s, though some authorities frown on these and prefer the traditional form hark back.
The eggcorn harp back (influenced by harp on) is occasionally found, but generally regarded as an error.
Originally a hunting command, literally “Listen! Go back!”, thence a term to refer to the process. In the 19th century US became used in a figurative sense “to go back to an earlier point or topic in a discussion”. In 20th century American usage used more loosely for references to the past (often historical past, rather than earlier in a given discussion), particularly nostalgic.