#Metoo is a movement within America's elite and within showbusiness, a Twitter hashtag, and communication tool used to elevate the visibility of those who claim they were recipients of non-consensual and/or unwanted sexual advances.
Adherents of #Metoo are greatly concerned with negatives of such sexual advances, including the potential illegality, as well as the potential to induce trauma in the recipient of the relevant sexual advances.
'Metoo' proponents have broadened the scope of 'problematic' sexual advances to include situations that are common, unavoidable, and not traumatic. Examples include: unwanted non-genital touch after mistaken sexual interest, bad dates, and regretted mutual sex acts. These situations will often lack malice, deviance, a decline of sexual reception, aggression, and the like by the alleged aggressor. Critics of #metoo often attack #metoo on the basis that the accusations filed at alleged abusers are often malicious or delusional post-hoc rationalization.
Reification of spurious assault victimhood claims is shown through the 'Metoo' usage of the phrase "Believe all women". Critics of this phrase, including self-described incel people (post-2017), point out that someone's social status or looks unfairly determine if they are accused of sexual assault. After all, if all that matters are the words of the accuser after the alleged abuse itself, accusers can and do use this power to conflate any or all displeasure with a person with 'sexual abuse'.
October 2017 New York Times article
The 'Metoo' hashtag gained viral popularity for years after an October 2017 New York Times article written by Jodi Cantor and Megan Twohey called Harvey Weinstein Paid Off Sexual Harassment Accusers for Decades. The article focused on a half-open secret in the entertainment business that American movie producer Harvey Weinstein was sexually abusive towards women who hoped to use him as a stepping stone into showbusiness. It also covered Harvey's attempted coverups of this half-open secret. For years after the article was published, there was an increase in prominent men in showbusiness being fired for allegations of sexual harassment.
Media outlets claim the shortened phrase "MeToo" was coined by Tarana Burke in 2006 as a therapeutic term for survivors of sexual violence.
The Twitter hashtag #metoo was used in reference to alleged sexual assault on Twitter prior to October 2017 sporadically. For example, one of the first usages of the #metoo hashtag in this way was from Twitter user ebaby_2841 in 2012. Other related mentions occur from Twitter user assault_survivr in 2016, and Twitter user AlbroMarie during August 2017.
Prior to October 2017, the #metoo hashtag was nonetheless mostly focused on topics other than sexual assault, and had not yet established itself as a recognized hashtag to use in reference to sexual assault. Given the frequency with which it was used, it's not surprising it was eventually applied as a hashtag in reference to sexual assault, especially as it's a common phrase in English.
Historical movement vs not
There is debate as to whether #metoo constitutes a large historical movement separate from Hollywood and Twitter. The only publicized march appears to have occurred in Hollywood, and hysteria about sexual assault was increasing in public before the Weinstein piece and the associated #metoo posts.