oversharern.
/ˈəʊvəˌʃɛːrə/, /ˌəʊvəˈʃɛːrə/, U.S. /ˈoʊvərˌʃɛrər/, /ˌoʊvərˈʃɛrər/ A person who reveals an inappropriate amount of detail about his or her personal life.
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1999 New Statesman 11 Oct. 17 No wonder..Blair got himself in such a muddle, getting too personal when we need him to be more political... Blair may be an over-sharer, but at least he thinks big.
2005 Harper's Bazaar 1 July 136 Spilling the most intimate details of your life to a stranger has never been chic. Here, a self-confessed oversharer explains why some people can't help themselves.
2014 Hoosier Times (Bloomington, Indiana) 19 Jan. (Herald-Times ed.) f5/1 Today's teens are notorious over-sharers on social media.
This is a new entry (OED Third Edition, December 2020; most recently modified version published online March 2022).
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n.1999