pp. Watching a TV show while talking to other people about that program online.
Other Forms
Where many other types of media use might be seen to be taking viewers away, chatterboxing can actually serve to reinforce more traditional television viewing habits.
—Sarah Martindale, “Viewers are changing TV for good,” The New Zealand Herald, August 29, 2013
Chatterboxing is particularly popular during soaps — almost 40,000 tweets are made during an episode of EastEnders. Juicy storylines can see tweets hit 6,000 a minute.
—Robert Wright, “Why we’ll never get fed up watching the telly,” Sunday Post, August 07, 2013
2012 (earliest)
Research conducted for TeleScope 2012, which looks at the UK's TV viewing habits, suggests that the trend of commenting via a second screen about a programme, or ‘chatterboxing’, is starting to grip the nation. A quarter of all adults (26%), and just under half (44%) of those aged under 35, say they have commented to others, online or via SMS, about a TV programme they have been watching.
—“TeleScope 2012 report reveals nation’s love for TV,” TV Licensing, March 15, 2012
This term is a portmanteau of the words chatter (what people generally do on social media) and box (a colloquialism for "TV set", which the OED dates all the way back to 1950).