This hashtag stands for Anti-Brexit Tactical Voting. It was started on January 12, 2018 by AC Grayling:
One of the issues it runs into is quite who counts as an anti-Brexit candidate deserving of such tactical voting support. Even Liberal Democrat Parliamentary candidate ranks in 2017 included at least one out and out pro-Leave person, whilst the bigger and pointed issue is with Labour.There are definitely anti-Brexit people in Labour. Their support of that position is to be welcomed and encouraged. But their leader is not only someone who for years voted with the Tory right wing Eurosceptics such as John Redwood but who also sacked Labour frontbenchers for voting against Britain leaving the single market.
Cross-party cooperation in various forms to oppose Brexit has much to commend it. Whether a cross-party anti-Brexit tactical voting campaign can overcome the controversies involved in deciding who to support is a much more open question.